                             PUBLISHED

                  UNITED STATES COURT OF APPEALS
                      FOR THE FOURTH CIRCUIT


                            No. 13-4136


UNITED STATES OF AMERICA,

                Plaintiff - Appellee,

           v.

DANILO GARCIA, a/k/a Donny, a/k/a Darreo,

                Defendant - Appellant.



Appeal from the United States District Court for the District of
Maryland, at Baltimore.    Catherine C. Blake, District Judge.
(1:11-cr-00569-CCB-1)


Argued:   March 20, 2014                    Decided:   May 15, 2014


Before GREGORY and KEENAN, Circuit Judges, and DAVIS, Senior
Circuit Judge.


Vacated and remanded by published opinion. Senior Judge Davis
wrote the opinion, in which Judge Gregory and Judge Keenan
joined.


ARGUED: Todd Michael Brooks, WHITEFORD TAYLOR & PRESTON LLP,
Baltimore, Maryland, for Appellant. Ayn Brigoli Ducao, OFFICE
OF THE UNITED STATES ATTORNEY, Baltimore, Maryland, for
Appellee. ON BRIEF: Erek L. Barron, WHITEFORD TAYLOR & PRESTON
LLP, Bethesda, Maryland, for Appellant.    Rod J. Rosenstein,
United States Attorney, OFFICE OF THE UNITED STATES ATTORNEY,
Baltimore, Maryland, for Appellee.
DAVIS, Senior Circuit Judge:

      Appellant Danilo Garcia, one of fourteen defendants named

in a superseding indictment returned by the grand jury in the

District of Maryland, appeals his convictions on five counts of

narcotics trafficking. After a severance, Garcia proceeded to

trial with two codefendants.

      The Government’s presentation at trial consisted primarily

of    two   types    of    evidence:     (1)   law    enforcement       testimony

recounting    observations      (and    describing    a    few   drug    seizures)

resulting from close surveillance of the physical comings and

goings of numerous coconspirators and targets, including drug

couriers;    and     (2)    audio      recordings     of    wiretapped     mobile

telephone    conversations      concerning     drug    supplies,      deliveries,

and   payments      therefor.   Over     defendants’       vigorous     objections

before and during trial, the district court permitted an agent

of the Federal Bureau of Investigation to testify as an expert

on coded drug-related conversations. The agent also testified as

a fact witness regarding aspects of the lengthy investigation.

      The jury convicted Garcia of drug conspiracy and of the

four substantive counts in which he was named, but it was unable

to reach a unanimous verdict as to his co-defendants and the

court declared a mistrial on those charges.

      On appeal, Garcia assigns error to the district court’s

admission of the decoding expert’s testimony, and the court’s

                                         2
denial   of   his   motion       for    judgment       of   acquittal      for    lack   of

sufficient evidence as to one of the substantive counts on which

he was convicted.

       Upon our careful review of the record, we hold that the

district court abused its discretion in its evidentiary rulings

as viewed in their totality. Specifically, we hold that, on this

record, safeguards adopted by the district court to avoid the

substantial risk of prejudice inhering in the jury’s receipt of

the decoding expert’s testimony were inadequate. Garcia timely

and repeatedly objected regarding the foundational sufficiency

and methodological reliability of the agent’s expert testimony,

and he specifically pointed to the risk of prejudice arising

from   the    agent’s     dual    capacity       as    both      an   expert     and    fact

witness. We are persuaded that, under the circumstances of this

case, neither the district court’s cautionary instructions to

the    jury   nor   its    sporadic       sustaining        of    some     of    counsels’

objections      adequately        mitigated           the   risk      of    substantial

prejudice.     Furthermore,       we     are     unable     to    conclude       that    the

missteps evident in this record were harmless. We reach this

conclusion     reluctantly,            because    the       district       court       tried

mightily to hew to the lines we have drawn in prior cases.

       Accordingly, although we discern no reversible error in the

court’s denial of the motion for judgment of acquittal, we hold

that the errors in the decoding expert’s testimony so infected

                                            3
the entire trial that we must vacate the judgment and remand the

case to the district court for further proceedings.

                                              I.

                                              A.

       On August 23, 2012, the grand jury returned a ten-count

superseding      indictment        against         Garcia       and   thirteen      others.

Garcia    was    named      in   the     following        five    counts:     Count     One,

conspiracy      to   distribute        more       than    one    kilogram    or    more   of

heroin, in violation of 21 U.S.C. §§ 841(a)(1), 846, from April

2009 to November 2011; Count Four, possession with intent to

distribute heroin on April 19, 2011 in violation of 21 U.S.C.

§ 841(a)(1); Count Five, possession with intent to distribute

100 grams or more of heroin on July 22, 2011 in violation of 21

U.S.C.    § 841(a)(1);           Count    Six,       possession       with    intent      to

distribute 100 grams or more of heroin on August 5, 2011 in

violation of 21 U.S.C. § 841(a)(1); and Count Seven, possession

with intent to distribute 100 grams or more of heroin on April

17, 2009 in violation of 21 U.S.C. § 841(a)(1).

                                              B.

       At the times relevant to this case, Garcia lived in New

York     City.       The     basic       theory          of     the   prosecution         was

straightforward:           (1)   coconspirator           Yoni    Rodriguez,       who   pled

guilty in the Eastern District of New York on narcotics charges

and testified against Garcia pursuant to a plea agreement, was

                                              4
Garcia’s      source      of   uncut    heroin   starting      sometime    in   2005,

selling to Garcia every few weeks or so, usually on consignment;

(2)    Garcia     would     distribute     the   heroin   in    Baltimore,      after

either transporting the drugs himself or via drug couriers from

New York; (3) Roy Lee Clay and Walter Lee Powell were Baltimore-

based middlemen who distributed quantities of heroin to other

dealers in the Baltimore area.

       In addition to the charge of knowing participation in the

thirty-month       drug     trafficking     conspiracy,     Garcia      was   charged

with       four   specific     instances    of   possession      with     intent   to

distribute        heroin,      the     circumstances   surrounding        which    we

summarize as follows:

            On April 17, 2009, a police officer observed
       Garcia exiting a white shuttle bus in the parking lot
       of a Baltimore travel plaza, carrying a brown paper
       bag. After Garcia saw the police officer, he reentered
       the bus, left the bag behind, and disclaimed any
       knowledge of or interest in the bag. The officer found
       a manicure set inside the bag, and inside the manicure
       set he discovered approximately 200 grams of heroin.

            On April 19, 2011, police observed Powell enter
       Kenya Salik Montgomery’s car and exiting after about
       two minutes. When, about two hours later, the police
       searched Montgomery, they found multiple baggies of
       heroin. 1



       1
        Garcia’s convictions on this and the two subsequent
incidents are based on an aiding and abetting theory, tracing
the drugs seized back to the heroin Garcia had obtained from
Rodriguez and then distributed to dealers in Baltimore.



                                            5
          On July 22, 2011, Garcia spoke by phone with
    Powell    and   instructed    Powell  to     travel   to
    Philadelphia. The FBI observed Powell when he arrived
    in Philadelphia and visited a certain neighborhood.
    After   remaining  in   Philadelphia  a   mere   fifteen
    minutes, Powell returned to Maryland (followed by law
    enforcement).    A  Maryland    State   Police   trooper
    conducted a traffic stop upon Powell’s entry into the
    state and the trooper recovered 143.7 grams of heroin
    from Powell.

         On August 5, 2011, FBI agents observed Nancy
    Feliciano at a travel plaza in Baltimore, leaving a
    van and getting into a cab. As she was getting into
    the cab, the agents approached her and searched her
    bags (with her consent). They found approximately 500
    grams of heroin and an index card which had Powell’s
    address and phone number. Later, while she was being
    detained in the custody of the FBI, Feliciano made a
    monitored call to Garcia and accused him of providing
    her with heroin.

    During   the   trial,   many        members   of   law   enforcement,

including FBI agents, U.S. Drug Enforcement Agency agents, and

state and local law enforcement officers, testified regarding

their surveillance and observations of the defendants together

in and around Baltimore, as well as in New York and once in

Philadelphia. Critically, much of the investigation entailed the

maintenance of wiretaps on four mobile phones used by Garcia,

two used by Clay, and two used by Powell. To a significant

extent, coded telephone conversations could be aligned with the

physical activities of the coconspirators.

                                   C.

    On August 30, 2012, the Government provided notice to the

defendants of its intent to call FBI Special Agent Carrie Dayton

                                   6
as an expert. The Government provided Agent Dayton’s resumé, and

stated that she “served as a monitor on the wiretaps, and will

testify as to the meaning of coded references in several of the

calls used by the conspirators, when discussing drug trafficking

over the phone.” J.A. 38. The Government’s disclosure showed

that Agent Dayton had worked as a paralegal and then, for more

than six years, as an attorney, before joining the FBI in May

2004. She thus had eight years of law enforcement experience by

the   time    of     trial.      She    initially      worked    in   the   FBI’s   Civil

Rights and White Collar Crime divisions, and joined the Safe

Streets      Task    Force       in    November    2007,    working     narcotics    and

violent crime investigations.

      Agent Dayton had “observed and [] recognize[d] patterns of

behavior consistent with narcotics trafficking” as a result of

her   participation         in    over    20     investigations.      J.A.    57.   Agent

Dayton was also involved in over five wiretap investigations,

during which she “listened to thousands of phone conversations

between individuals who participated in or were suspected of”

drug trafficking conspiracies and other crimes. Id. Agent Dayton

attested      that    she     has      “gained     a   working    knowledge    of    drug

language and terminology commonly used by drug dealers” and that

she was “aware of the technique and [has] heard hundreds of

conversations in which narcotics traffickers employ code words



                                               7
to disguise discussions” about drugs. Id. Agent Dayton had not

previously testified as an expert on coded drug language.

     Days before the commencement of trial, defendants moved to

exclude the testimony of Agent Dayton on the basis that the

Government failed to provide a sufficient expert disclosure in

conformity with Federal Rule of Criminal Procedure 16(a)(1)(G).

On October 2, 2012, after opening statements, the district court

denied   the     motion.   Referring     to    whether     the   Government    had

provided     a    sufficient    outline       of   Agent    Dayton’s     proposed

testimony, the court stated that,

     it appears to me that when a disclosure says she is
     going to testify to coded references in transcripts of
     calls related to narcotics, clearly, based on the
     openings, counsel have understood the government’s
     theory to be that references to things like cars and
     perhaps girls are in fact references to drugs, that it
     does not take any great additional leap of inference
     from there to figure out what Special Agent Dayton is
     going to testify to.

J.A. 177-78. Accordingly, the district court held that there was

no   prejudice      to   the   defense    “even    if    there   was    something

deficient in the disclosure.” J.A. 178.

                                         D.

     On eighteen separate occasions during six days over the two

week trial, the Government called and re-called Agent Dayton to

the stand to testify on the coded language used in recorded

wiretapped       calls   between   the    conspirators.      From      the   start,

defense counsel voiced vigorous and repeated objections to Agent

                                         8
Dayton’s qualifications as an expert witness in decoding drug

traffickers’           language.        Counsels’       objections           continued    as    the

presentation of her testimony unfolded, specifically based on

Agent Dayton’s failure to conform to recognized methodologies

for such testimony and the Government’s failure to explicitly

demonstrate the existence of an acceptable foundation for many

of her specific interpretations. 2 The district court generally

overruled          the     defense       objections         (while       sustaining       some),

holding that Agent Dayton had “sufficient experience to meet the

standard to be able to offer opinion testimony that might be

helpful to the jury based on . . . her five years of experience,

dozens      and     dozens    of     shifts      on     wiretaps,        and    thousands       and

thousands         of     calls,    as    well    as     her      conversations       with      both

cooperators,           witnesses,       and     other    law      enforcement        officers.”

J.A. 320.

       Manifesting a deep familiarity with this Circuit’s settled

guideposts          with     regard       to     this       type        of     testimony,       the

experienced         judge    identified         early       on    two    potential       problems

with       Agent    Dayton’s       testimony:         (1)     the   need       to   distinguish

between her lay fact testimony based on her personal knowledge,

on the one hand, and her expert opinion testimony based on her

       2
        It appears, and we presume, that there was an
understanding at trial that an objection made by any one of the
three defense counsel was joined by the others.



                                                 9
training and investigatory experience, on the other; and (2)

ensuring that she was testifying on the basis of her experience

and expertise in coded language, and not simply repeating what

cooperators or witnesses told her.

        This second point was especially salient because, as Agent

Dayton conceded during voir dire in the presence of the jury,

she   had       debriefed    several      cooperating       coconspirators         in    this

very case, including at least one of whom speaks only Spanish,

Garcia’s        supplier,    Yoni   Rodriguez.           Thus,   it     was   particularly

apparent at the start of trial (especially as Agent Dayton had

never     before     testified      as    an    expert)     that      special     care   was

required to expose to the jury (and to the defense) the true

bases for Agent Dayton’s “opinions.”

      In any event, cognizant of these perils, the district court

issued      a    cautionary    instruction          to    the    jury    prior    to    Agent

Dayton’s        testimony.    It    informed        the   jury    that    a   distinction

would     be     made   between     the    agent’s        fact    and    expert    opinion

testimony, and that it would not be appropriate for Agent Dayton

to testify to the meaning of a word simply because a witness

told her its meaning. 3


      3
          The district court told the jury:

     The fact that I am determining that she has sufficient
     qualifications to offer you opinion testimony, again,
     I’m not deciding what weight you give her testimony or
(Continued)

                                               10
     how you evaluate it. It’s up to you, as I said at the
     beginning, to pay attention to her experience, and her
     background, and training, and the reasons that she
     gives you for any opinion that she may offer, and then
     you decide what weight to give it.

     The other thing I want to say is that she may also be
     offering more fact testimony, just an ordinary lay
     witness or fact testimony about certain matters, and I
     have asked counsel to be clear in their questions
     which is which.

     To the extent she is offering you just fact testimony,
     something she actually saw or heard, that should be
     clear. To the extent it’s her opinion, based on her
     experience and review of these calls, we’ll make that
     clear as well, and she will be asked to explain what
     the reason for her opinion is.

     ***

     But I want to be clear, any opinion that she may be
     offering you about these calls is not to be based on
     what somebody, who may or may not have been involved
     in this case, told her. Mr. Jones said that means
     cocaine, for example.

     That’s not the basis of her opinion. That’s not
     appropriate. She is not here to just repeat something
     that somebody may or may not have said to her when we
     don’t have that person here for you to listen to, and
     make your own opinion about credibility.

     So her opinion, and we will be careful to limit it to
     that, is to the extent she has got a basis, on her
     experience and training and everything she told you
     about yesterday, to have a reason to believe that a
     certain reference means something that is not apparent
     on its face, she will explain that, and it is up to
     you to evaluate it again. But it is not because it is
     based on anybody that she may or may not have talked
     to in this case about the meaning of certain words.

J.A. 326-28.



                               11
      In   the    course   of     the    investigation    in     this   case,

investigators had monitored in whole or in part more than 20,000

calls; the Government considered 1,928 conversations pertinent

to the investigation and it played approximately 211 of such

monitored calls, many in Spanish, during the trial. Agent Dayton

testified repeatedly over the course of six days, “interpreting”

words in nearly half the calls played before the jury, including

the   English    translations     of    the   conversations    occurring   in

Spanish.   The   defense   made   countless     objections     regarding   the

melding of her fact and expert opinion testimony, the agent’s

failure to adhere to her professed methodology, and the absence

of foundations for many of her specific interpretations. While

the district court sustained some objections, 4 for the most part,


      4
       One notable objection early in Agent Dayton’s testimony
came after she testified as follows after listening to a wiretap
recording:

      AGENT DAYTON: In the second line of Mr. Coley
      speaking, wherein he says I owe you 200, he is
      actually talking about $2,000. In that same sentence,
      when he says I have a hundred and 50 for you, he is
      actually referring to $1500.

      GOVERNMENT: Are these amounts in reference to, based
      on your expert opinion, in reference to, these dollar
      amounts, in reference to anything in particular?

      AGENT DAYTON: These are in reference to – Mr. Powell
      has supplied Mr. Coley with heroin, and Mr. –

J.A. 364. Defense counsel objected at this point, arguing that
Agent Dayton had gone “beyond the spectrum of what she is
(Continued)

                                        12
capable of doing” by “opin[ing] as to . . . the general nature
of this particular conversation, and what the conversation
means[.]” J.A. 365. Counsel further argued that “there has been
no foundation or no explanation as to how she came to some of
these conclusions. She is . . . just saying this is what this is
about, and we are listening to these calls isolated, without any
proper context.” Id.

     The district court astutely summed up the issue:

     The problem is that [Agent Dayton] just said, in
     response to your question about how she knows that the
     200 is a reference to 2,000, she just said because I
     know that Mr. Coley supplied heroin to Mr. Powell.
     That is not based on her expertise or the coded
     language. She is using fact information that she has
     gained otherwise, which is not appropriate.

J.A. 366-67. The prosecutor responded by stating that she was

     trying to stay away from the objections raised by
     counsel regarding her, Special Agent Dayton testifying
     about what she learned from persons in this case, but
     it does form the basis, part of the basis of her
     opinion. I can certainly elicit that, but then I would
     think there’s going to be an objection to her
     testifying about that being the basis of her opinion.

J.A. 367-68.

     The district court ultimately sustained the objection (as
it did some others along the same vein) after the prosecutor
further stated that “[r]egarding the numbers that Mr. Coley
uses, she is in part relying on the investigation which showed
that Mr. Coley and Mr. Powell met,” to which the district court
responded, “Then it’s hearsay and it’s not an expert opinion.”
J.A. 369.

     The fact that the prosecutor hesitated to lay an adequate
foundation for Agent Dayton’s opinions for fear of objection
from defense counsel speaks volumes to the impermissible basis
for that foundation. See also J.A. 375 (district court sustained
objection because Dayton’s testimony that “cousin” meant a
specific individual was on the basis of her personal knowledge,
(Continued)

                               13
it   admitted      the    bulk     of   Agent       Dayton’s    testimony,         which   was

often     elicited       through    a   prosecutor’s           insertion     of     a   simple

prologue     to      a     question:       “Agent       Dayton,       in     your       expert

opinion . . . ”            or       “Agent           Dayton,         based         on      your

expertise . . . .” 5

      As    mentioned,       Agent      Dayton’s       testimony       was    interspersed

with the testimony of other investigators. After a 12-day trial,

the jury returned a verdict of guilty on all five counts in

which Garcia was named (one count of conspiracy to distribute

heroin, and four counts of possession with intent to distribute

heroin),     but     a    mistrial       was        declared    as    to     the    two    co-

defendants. The district court entered judgment and sentenced

Garcia to 188 months on all counts, to run concurrently.

      Garcia       timely       appealed        and     we     exercise       jurisdiction

pursuant to 28 U.S.C. § 1291.




not on expertise); J.A. 452 (district court overruled objection
based on lack of foundation despite term only heard in this
investigation); J.A. 461 (district court overruled objection and
advised Agent Dayton to “rely on what [she] learned prior to
this   investigation”);  J.A.   573  (district   court overruled
objection where foundation for interpretation was that “Mr.
Garcia uses the term the [sic] G. We see him use that in other
calls on this wire to refer to a thousand dollars.”).
      5
       As we discuss infra n.10, this formulaic approach hardly
cured the problems that the district court anticipated and that
we find actually to have arisen in the course of trial.



                                               14
                                II.

     Garcia presents a number of arguments on appeal regarding

the admissibility of Agent Dayton’s expert testimony. While we

find that the district court did not abuse its discretion in

qualifying Agent Dayton as an expert, we are persuaded that the

district court failed to adequately safeguard against a strong

likelihood of jury confusion between Agent Dayton’s testimony as

an expert witness and as a fact witness, and in ensuring that,

in her capacity as an expert witness, Agent Dayton “reliably

applied” her methodology and adequately set forth the underlying

permissible foundations for her opinions. 6


     6
       Garcia also challenges the sufficiency of the evidence on
Count 4 (the April 19, 2011 Powell/Montgomery transaction), for
possession of heroin with intent to distribute. Garcia’s
conviction on that count, as on two of the other three of the
substantive counts, was based on an aiding and abetting theory.
We find the evidence was sufficient to prove beyond a reasonable
doubt that Garcia “knowingly associated himself with and
participated in the criminal venture.” United States v.
Winstead, 708 F.2d 925, 927 (citing Nye & Nissen v. United
States, 336 U.S. 613, 619 (1949) (further citations omitted));
see also Rosemond v. United States, 134 S.Ct. 1240, 1245-51
(2014)    (explaining    aiding     and    abetting     principles).
“[P]articipation in every stage of an illegal venture is not
required, only participation at some stage accompanied by
knowledge of the result and intent to bring about that result.”
United States v. Burgos, 94 F.3d 849, 873 (4th Cir. 1996) (en
banc) (internal citations omitted). Rodriguez testified that he
sold heroin to Garcia, which Garcia then sold to Powell for
further   distribution   to   customers    like   Montgomery.    The
relationship between Garcia and Powell was established by the
wiretap calls, as well as surveillance of Garcia and Powell
together.   Furthermore,   on   the   very   day    Montgomery   was
apprehended in possession of heroin, a conversation between
(Continued)

                                15
                                 A.

     Garcia   contends   that   the   district   court   abused   its

discretion in qualifying Agent Dayton as a decoding expert in

the first place, and that therefore the district court erred in

admitting her testimony, without regard to the special dangers

inherent in the use of decoding experts. 7 We disagree.




Garcia and Powell was recorded on Montgomery’s voicemail, in
which Garcia and Powell discussed their drug trafficking plans
as interpreted by Agent Dayton.

     We note that even though we find error in the manner in
which Agent Dayton’s testimony was presented to the jury, we
must still consider that testimony in reviewing the denial of a
Rule 29 motion for judgment of acquittal. United States v. Diaz,
300 F.3d 66, 77 (1st Cir. 2002); see also United States v.
Horton, 693 F.3d 463, 482 (4th Cir. 2012) (Davis, J.,
concurring) (discussing the necessity of ruling on appellate
claims of evidentiary insufficiency even in the context of trial
error requiring reversal).

     Of course, we intimate no view as to whether upon a retrial
the Government will adduce sufficient evidence to support
conviction as to any count of the indictment.
     7
        Garcia also argues that the government’s pretrial
disclosures regarding Agent Dayton were insufficient. Federal
Rule of Criminal Procedure 16(a)(1)(G) requires that “[a]t the
defendant’s request, the government must give to the defendant a
written summary of any testimony that the government intends to
use under Rules 702, 703, or 705 of the Federal Rules of
Evidence during its case-in-chief at trial. . . . The summary
provided under this subparagraph must describe the witness’s
opinions, the bases and reasons for those opinions, and the
witness's qualifications.” Garcia maintains that the information
contained in the Government’s September 24, 2012 letter, and
Dayton’s attached resumé, was insufficient to meet this
requirement.

(Continued)

                                 16
     We review a district court’s decision to qualify an expert

witness, as well as the admission of such testimony, for abuse

of discretion. United States v. Wilson, 484 F.3d 267, 273 (4th

Cir. 2007). “A court abuses its discretion if its decision is

‘guided by erroneous legal principles’ or ‘rests upon a clearly

erroneous factual finding.’” United States v. McLean, 715 F.3d

129, 142 (4th Cir. 2013) (quoting United States v. Johnson, 617

F.3d 286, 292 (4th Cir. 2010)).

     The Federal Rules of Evidence provide that a

     witness who is qualified as an expert by knowledge,
     skill, experience, training, or education may testify
     in the form of an opinion or otherwise if:

     (a) the expert’s scientific, technical, or other
     specialized knowledge will help the trier of fact to
     understand the evidence or to determine a fact in
     issue;

     (b) the   testimony   is   based   on    sufficient    facts   or
     data;

     (c)   the  testimony   is   the         product   of   reliable
     principles and methods; and

     (d) the expert has reliably applied the principles and
     methods to the facts of the case.




     In light of our ultimate ruling on the propriety of Agent
Dayton’s testimony, we need not address the adequacy of the
Government’s compliance with Rule 16(a)(1)(G).



                                  17
Fed. R. Evid. 702. In the advisory committee notes following the

2000 amendments to the Rule, there is a pointed discussion on

the use of hybrid witnesses at issue here:

     For example, when a law enforcement agent testifies
     regarding the use of code words in a drug transaction,
     the principle used by the agent is that participants
     in such transactions regularly use code words to
     conceal the nature of their activities. The method
     used by the agent is the application of extensive
     experience    to   analyze    the   meaning    of  the
     conversations. So long as the principles and methods
     are reliable and applied reliably to the facts of the
     case, this type of testimony should be admitted.

Fed. R. Evid. 702 advisory committee’s note (emphasis added).

     Garcia’s    argument      that    Agent       Dayton’s       experience    was

insufficient    to   qualify   her    as    a    decoding    expert,    given   the

emphasis on experiential knowledge for these types of experts,

misses the mark. We think Fourth Circuit law in this area is

reasonably   clear.    Wilson,   484       F.3d    at    267;   see   also   United

States v. Galloway, --- F.3d ---, ---, No. 12-4545, 2014 WL

1424939, at *4-6 (4th Cir. April 15, 2014) (applying plain error

review).

     In Wilson, we reviewed the district court’s admission of a

police   investigator’s     expert    and       fact    witness   testimony;    the

investigator’s expertise was in “decipher[ing] the intercepted

communications that were recorded via wiretap” as they “were

full of drug slang and street code[.]” Id. at 273. We found no

abuse of discretion in the district court’s admission of the


                                       18
expert    evidence.      There,     the    expert’s    qualifications      included

nine   years     spent     mostly     investigating      drug   traffickers     and

attendance in training programs offered by the DEA, training

which specifically included exposure to drug traffickers’ use of

coded language. Id. at 275-76.

       While Agent Dayton certainly has less experience than the

witness in Wilson, and had never before this case been qualified

as an expert, the precise number of years of an investigator’s

experience, or the number of investigations on which she has

worked, is not necessarily dispositive. Every expert has a first

time. It is the quality of Agent Dayton’s experience, especially

her exclusive focus on narcotics trafficking for the preceding

five   years,    on     which   the   district    court   properly    focused     in

finding her qualified as a decoding expert.

       Agent Dayton’s job required her “to work in close proximity

with drug users on a daily basis,” United States v. Baptiste,

596 F.3d 214, 218 (4th Cir. 2010); she had “monitored dozens of

wiretaps, listening to thousands of intercepted phone calls in

the process,” United States v. Mack, 495 Fed. App’x 359, 364

(4th   Cir.     2012)     (internal       quotations   omitted);     and   in   the

“course    of    [her]      extensive       experience     dealing    with      drug

traffickers and confidential informants, [] was able to learn to

understand the vernacular used by drug traffickers.” Wilson, 484

F.3d at 275-76.

                                           19
      Garcia also takes issue with Agent Dayton’s explanation of

her methodology. The defendants in Wilson similarly challenged

the investigator’s methodology, claiming that it was “neither

sufficiently explained nor reliable.” 484 F.3d at 273. Agent

Dayton explained her methodology as follows: “I think we see

some common [terms], receipts or paper or clothing terminology.

So[me] of it’s common and some is just in the context of the

conversation. As you listen to the conversation, the language

used doesn’t make sense, and sometimes they switch up code so

the context is all wrong.” J.A. 285. This is very similar to the

witness’s explanation in Wilson. 484 F.3d at 275 (explaining

that the expert relied on “the context of the call” to see the

“pattern   that   develops.”).      While     this    explanation    is   not   as

coherent as we might wish, given the deferential standard of

review applicable here, it passes muster as an articulation of

an accepted methodology. Agent Dayton also based her opinions in

part on conversations she had with witnesses in other cases who

“would talk about the drug shops and the language that was used

to communicate with one another.” J.A. 294.

      Given the similarity between Agent Dayton’s explanation of

her   methodology   and    the    methodology        we   found   acceptable    in

Wilson,    we   cannot    say    that   the   district      court   abused      its

discretion in qualifying Agent Dayton as an expert. As we have

recognized, “[a] ‘trial judge must have considerable leeway in

                                        20
deciding     in    a    particular    case    how   to    go    about    determining

whether particular expert testimony is reliable.’” Wilson, 484

F.3d at 273 (quoting Kumho Tire Co. v. Carmichael, 526 U.S. 137,

152 (1999)). Agent Dayton met the requirements for testifying to

the meaning of coded language recorded from the coconspirators’

calls, and adequately explained her methodology. Accordingly, no

abuse of discretion infects the district court’s determination

to accept Agent Dayton as a decoding expert.

                                         B.

      Despite      the    district    court’s    careful       attention    to   Agent

Dayton’s credentials as a decoding expert, however, we hold that

the agent’s testimony was fraught with error arising from the

problems     the    district    court    itself      identified     early    in    the

trial:     the     conflation    of     Agent    Dayton’s       expert     and    fact

testimony, particularly her reliance on her knowledge of the

investigation to support her coding interpretations; her failure

to   apply   her       methodology   reliably;      and   last,    her   failure    to

state on the record an adequate foundation for very many of her

specific     interpretations.         Moreover,      because       Agent    Dayton’s

testimony was so extensive and most likely highly influential in

the jury’s evaluation of the Government’s case against Garcia,

we are constrained to hold that these flaws deprived Garcia of a

fair trial, i.e., that the missteps were not harmless, and thus

require vacatur of Garcia’s convictions.

                                         21
                                           1.

      Garcia contends that there were inadequate safeguards to

protect the jury from conflating Dayton’s testimony as an expert

and fact witness. We agree.

      We have recognized that individuals who testify as expert

and fact witnesses can cause jury confusion, and such a manner

of proceeding is only “acceptable where ‘the district court took

adequate steps . . . to make certain that [the witness’s] dual

role did not prejudice or confuse the jury.’” Baptiste, 596 F.3d

at 224 (quoting Wilson, 484 F.3d at 278 n.5) (alternations in

original). Such safeguards might include requiring the witness

to   testify    at    different     times,       in    each   capacity;      giving   a

cautionary instruction to the jury regarding the basis of the

testimony;     allowing    for    cross-examination           by   defense   counsel;

establishing a proper foundation for the expertise; or having

counsel ground the question in either fact or expertise while

asking the question. Id. at 224 (citing Wilson, 484 F.3d at 278

n.5 and United States v. Farmer, 543 F.3d 363, 370-71 (7th Cir.

2008)).

      While a cautionary instruction might be sufficient in many

instances      to    protect     against        jury   confusion     and     resulting

prejudice, see Galloway, --- F.3d at ---, 2014 WL 1424939 at *5-




                                           22
6, 8   we     are    persuaded      that      the    instruction       given     here   was

insufficient to mitigate the potential for prejudice.

       The district court represented to the jury that Government

counsel would “be clear in their questions” whether they were

asking Agent Dayton to testify based on the facts versus her

expertise.          J.A.   327.    Despite      this    direction,      the    Government

failed to do so: there were repeated instances of Agent Dayton

moving back and forth between expert and fact testimony, with no

distinction in the Government’s questioning or in Agent Dayton’s

answers. And this was true despite the fact that the Government

recalled Agent Dayton to the stand eighteen times. In light of

the     court’s       earlier      assertion         that    counsel     would    clearly

distinguish         the    two    types    of    testimony,     the    jury    reasonably

might       have    assumed      that   all     of   Agent    Dayton’s    testimony     in

response to questions asking for her expert opinion was indeed

based on her decoding expertise.


       8
       In Galloway, the district court repeatedly instructed
counsel to be clearer in the demarcation between fact and lay
opinion testimony. See, e.g., Joint Appendix at 472-75, 572-73,
2014 WL 1424939, ECF No. 59. There, the district court generally
did not allow the expert to testify about the meaning of
individual words in particular conversations, but rather only
permitted the expert to testify whether, in his expert opinion,
coded language was used in the conversation at issue as a
general matter. There was little opportunity, then, for the
expert’s participation in the underlying investigation to
encroach upon, enhance, or improperly bolster the expert
testimony of that witness.



                                                23
       Our     review       of       the     record       reveals         multiple         occasions    in

which the Government prompted Agent Dayton to assert information

garnered      from        her    participation             in    the      investigation,            having

nothing       to     do     with       her     ostensible            decoding         expertise.       For

instance, in response to a question about any coded language on

the    call        “based       on     [her]     expertise,”              Dayton       answered      that

“[w]hen Mr. Powell uses the term show time, he is letting Mr.

Coley know he has heroin.” J.A. 371-72. Immediately after this

exchange,       without          any       further     explanation             of    the     term   “show

time”    or        warning       that        they     were      shifting            away    from    Agent

Dayton’s expertise to her factual knowledge, the prosecutor then

asked Agent Dayton: “Now how were Ms. Montgomery and Mr. Coley

identified as the participants in the calls that we’ve just seen

or heard?” J.A. 372. Agent Dayton’s response had nothing to do

with    her     expertise,             and    everything             to   do    with       her     factual

knowledge as an investigator in this case.

        It is apparent even from the Government’s briefing that

Agent Dayton used her personal knowledge of the investigation to

form    (not       simply       to     “confirm”)         her    “expert”            interpretations:

“Special      Agent       Dayton        also    looked          to    the      actual       seizures   of

heroin in this case to form the basis of her expert opinion.”

Appellee Br. 46 n.8.

        Notably, the Government points to a call in which Agent

Dayton interpreted “a hundred forty five point,” to mean “145

                                                     24
grams of heroin,” J.A. 885, which is confirmed with evidence

that       police    seized        145    grams     of    heroin      from    one    of       the

coconspirators on the call. But unlike the display of expertise

we   endorsed       in     United    States       v.    Johnson,     this    is    not    Agent

Dayton “applying [her] expertise, derived over many years and

from multiple sources, to interpret the transcripts of phone

conversations.” 587 F.3d 625, 636 (4th Cir. 2009). Instead, this

exemplifies occasions, and we discern many of them, in which

Agent      Dayton     simply       substituted         information    gleaned       from      her

participation         in    the     investigation         (including        post-indictment

debriefings of participants in the conspiracy) for ostensible

expertise. 9        For    this,    the    Government       need     not    have    called      a

“decoding” expert at all: simply pointing to the seizure of 145

grams of heroin, and then the repeated mention of “145” in this

call clearly would have been enough for any juror to make the

connection. But cloaking this connection in the guise of expert

testimony goes beyond what is contemplated under Federal Rule of

Evidence 702, which requires an expert to “reliably appl[y] the

principles      and       methods”       for   which      she   was    qualified         as    an


       9
       But even if she did not, the record is bereft of evidence
that she did not, due largely to the Government’s deliberate
failure to lay an adequate foundation for many of her opinions.
For our purposes, this absence of clarity as to the source of
her opinions amounts to the same thing as if she had no
expertise at all.



                                               25
expert. The Rule contemplates that an expert’s opinion testimony

will    be   “helpful   to   the   jury,”   not     merely   helpful    to   the

prosecutor     as   transmutations    of    simple    fact   testimony.      Cf.

Wilson, 484 F.3d at 276-78 and nn.4 and 5 (emphasizing need for

the district court to enforce methodological, foundational, and

helpfulness standards in admitting testimony of decoding expert,

and    affirming    convictions    under    plain    error   review    because,

despite district court’s erroneous admission of some testimony,

expert’s “methodology was reliably applied in the vast majority

of instances”) (emphasis added).

       This issue actually surfaced early in the trial. During the

Government’s voir dire of Agent Dayton in its effort to satisfy

the district court that the agent had genuine “expertise,” the

following colloquy occurred:

       GOVERNMENT:   Special  Agent   Dayton, regarding the
       wiretaps in this case, you have spoken to at least
       three members of the organization?

       DEFENSE: Objection, Your Honor, leading.

       THE COURT: Overruled.

       GOVERNMENT: Is that correct?

       AGENT DAYTON:    At least three, maybe more.

       GOVERNMENT: Maybe more. To confirm your understanding
       of the coded language used in this case?

       AGENT DAYTON: Yes.




                                      26
J.A. 296. Later, the defense had Agent Dayton identify the three

coconspirators       whom    she    had   debriefed,     and     she   stated   the

following:       “Diego Amparo, Yoni Rodriguez, Nancy Feliciano. I’m

trying to think of the other folks who have come in. I’m sorry.

Those are the three I believe we discussed code with.” J.A. 298.

In   light      of   this   significant        ingredient   in    Agent   Dayton’s

expertise, it was incumbent upon the Government to demonstrate

that    Agent    Dayton     was   not   merely    channeling     information    and

statements by non-testifying participants in the conspiracy into

the trial record. 10


       10
        In what is truly a curious argument, the Government
contends that because Agent Dayton “did not offer any expert
opinion regarding any calls by Yoni Rodriguez, and discussed
only a handful of calls involving Diego Amparo and Nancy
Feliciano,” Appellee Br. 46, there should be no concern about
Agent Dayton’s ostensible methodologically sound expertise.
Exactly   the  opposite  is   true.  There  is   no  discernible
correlation favorable to the Government between Agent Dayton’s
debriefing of coconspirators whose calls she interpreted and
those whose calls she did not interpret. The point is that it
was one conspiracy and there is no reason to believe that
Rodriguez, in particular, would have been unable to provide
substantial information concerning the operational details of
the conspiracy. And in fact, he did so in his testimony,
including “interpretations” of many of the same words Agent
Dayton interpreted. In short, the funneling of fact information
learned from those inside the conspiracy and the offer of expert
testimony was melded in a highly prejudicial manner in this
case.

     Relatedly, the Government seems earnestly to contend that
simply by including in its questions to Agent Dayton that the
agent answer only based on her “expert opinion” somehow
insulates the agent’s testimony from ordinary scrutiny under the
settled principles of Daubert v. Merrell Dow Pharm., Inc., 509
(Continued)

                                          27
      In Johnson, we expressed wariness over this exact problem,

and cautioned against “[a]llowing a witness simply to parrot

out-of-court testimonial statements of cooperating witnesses and

confidential informants directly to the jury in the guise of

expert   opinion”     as   it   “would   provide   an   end   run   around

Crawford.” 587 F.3d at 635 (internal quotation marks omitted).

In   Crawford   v.   Washington,   the   Supreme   Court   held   that   the

Confrontation Clause permits the introduction of “[t]estimonial

statements of witnesses absent from trial . . . only where the

declarant is unavailable, and only where the defendant has had a

prior opportunity to cross-examine.” 541 U.S. 36, 59 (2004). Of

the three co-conspirators Agent Dayton specifically identified

as contributing to her “understanding of the coded language used

in this case,” though by her own admission there may have been

more than three, only one testified at trial. We explained that

in the face of such risks, “[t]he question is whether the expert

is, in essence, giving an independent judgment or merely acting



U.S. 579 (1993), and its progeny and Rule 702. The Government is
wrong. It is bootstrapping of the worst kind to suggest to a
jury that it should believe that everything a witness says is
based on expertise gained from independent knowledge and
experience in the absence of a record demonstrating as much. We
would never permit counsel calling a physician or an engineer or
an expert tightrope artist to proceed in such a manner; there is
scant  reason   we   should  allow   a  prosecutor  calling  law
enforcement officers as experts to do so.



                                    28
as a transmitter for testimonial hearsay.” Johnson, 587 F.3d at

635.

       Here,    we   cannot       say   that       Agent     Dayton      was    giving     such

independent     judgments.         While      it   is   true      that    she    never     made

“direct reference to the content of [her] interviews,” id., this

could just as well have been the result of the Government’s

failure    to     elicit      a     proper     foundation         for     Agent    Dayton’s

interpretations. See J.A. 367-68; supra 12-14 n.4 (discussing

the prosecutor’s statement in response to an objection for lack

of    foundation,     that        she   was    “trying       to   stay     away    from    the

objections raised by counsel regarding her, Special Agent Dayton

testifying about what she learned from persons in this case, but

it does form the basis, part of the basis of her opinion. I can

certainly elicit that, but then I would think there’s going to

be an objection to her testifying about that being the basis of

her opinion.”).

       Although      Agent    Dayton       asserted        that    her     after-the-fact

debriefing of coconspirators who had commenced cooperating with

the     prosecution        served        to        “confirm[]”           her     independent

assessments of the meanings to be attributed to many words, it

was imperative that the trial record demonstrate this sequence

with a measure of clarity. The record is devoid of evidence that

this    was,    in    fact,       the   sequence        of    Dayton’s         analysis,     to

Garcia’s prejudice.

                                              29
                                                 2.

        We    find        a    second,        equally      fundamental        flaw     in     the

presentation         of       Agent   Dayton’s        testimony    in   addition       to     the

above        infirmity:         the   lack      of      foundations      laid        for     each

interpretation testified to, so much so that we are compelled to

conclude       that   the       record    fails       to   demonstrate        the    requisite

reliability          in       Agent    Dayton’s         execution       of     her     claimed

methodology. Call Number 214, referenced above regarding Agent

Dayton’s       interpretation            of     the     term    “show    time”        to     mean

“heroin,” is an illustrative example of the utter absence of any

foundation        for         more    than      simply      a     few    of     her        coding

interpretations. No further explanation regarding the term “show

time” was given – Agent Dayton did not explain that she had seen

that term used in this investigation, let alone in her previous

experience. There was no explanation as to what, in the context

of the call or otherwise, led her to believe that “show time”

meant “heroin.” 11


        11
       And even where Agent Dayton stated (as she did from time
to time) that a term had been seen in other calls during the
investigation, this assertion alone is no explanation for the
threshold interpretation. Indeed, the fact that the expert has
heard a term in other calls during the investigation does not,
by itself, explain how the expert knows what the term means. As
discussed in text regarding the “interpretation” of numerals,
the same term means different things at different times, even in
the same conspiracy. The Government failed to show how Agent
Dayton reliably leapt these lexicographical crevasses.



                                                 30
        One   significant       indication      of    Agent     Dayton’s     failure     to

adequately explain her methodology is her lack of explanation as

to how her methodology was affected by the fact that many of the

telephone      conversations        were     in      Spanish.        As   Agent    Dayton

explained, the context of a conversation is an important factor

when decoding suspected drug language. Although the jury heard

English language transcripts of the actual call recordings read

aloud, Agent Dayton provided no insight as to what steps, if

any, she took to ensure that the context of the conversation was

not lost or meaningfully altered in the process of translating

the calls from Spanish to English.

      The     problem    with     this    lack       of    foundation      and    reliably

applied methodology becomes even more apparent when examining

the     instances        when     Agent      Dayton’s          interpretations          are

inconsistent with each other. For example, she testified that

the use of “2” by the conspirators means either $200, J.A. 600,

or    $2,000,     J.A.     533.     In     fact,          in   the    explanation       for

interpreting “2” to mean “$2,000,” Agent Dayton testified that

drug traffickers “drop zeros [sic] to make the numbers sound

smaller.” Id. This would be an explanation for both of these

interpretations, and there is no indication in the record why

Agent     Dayton’s      expert     methodology            reasonably      leads   her    to

conclude that the same term means $200 in one instance, but

$2,000 in another.

                                           31
      A like inconsistency is seen with regard to the number “5”:

Agent Dayton first interprets this as “$5,000,” J.A. 854, and in

another instance as 500 grams of heroin, J.A. 925. While there

might    very     well    be   an     explanation      for    this,      based    on    Agent

Dayton’s expertise, she did not explain it at trial to the jury,

and it is not evident from the record what that explanation

would be.

      In another instance, Agent Dayton testified that, in her

expert opinion, “590” was code for 590 grams of heroin. J.A.

774. She gave no explanation for the basis of this opinion, and

importantly, her testimony that the number 590 actually meant

590 is inconsistent with the only methodology she offered for

how she decoded the numbers heard in the calls: that the dealers

spoke in code about numbers by dropping the zeroes.

      In   several        instances,        Agent    Dayton    “decoded”         words    and

phrases    that     needed       no   expert       translation      at    all    since   the

meaning     was    either      apparent        on    its   face     or     apparent      with

contextual        information         that     any    fact     witness          could    have

provided.        See, e.g., J.A. 406-09 (testimony that the phrases

“first one” and “second one” were code for different deliveries

of   heroin);      J.A.    410,       412    (testimony      that    the    phrase      “over

there”     was    code     for      Baltimore);       J.A.    854,       857,    875,    1190

(testimony that the word “stuff” was code for heroin); J.A. 940

(testimony that the word “number” was code for price in the

                                              32
phrase “the number they gave me was too high.                    We’re, we’re on

the number he told me.”); J.A. 958 (testimony that the word

“number” was code for price in the phrase “The important thing

is the quality and a good number.”)                  This purported “decoding”

of language that did not actually need decoding casts further

doubt   on     whether    Agent       Dayton   was     reliably     applying      her

methodology. Unlike in Wilson, where we found that the expert

“applied      his    methods    and    principles      reliably     in    the    vast

majority of his testimony,” 484 F.3d at 277, we cannot say the

same is true of the record before us here. The record is replete

with instances of Agent Dayton providing no explanation for her

interpretation, other than a token reference to her expertise in

the Government’s framing of questions. While the district court

was appropriately careful in its initial examination of Agent

Dayton’s qualifications to testify as an expert, it failed to

maintain its “gatekeeper” role throughout that testimony, Gen.

Elec.   Co.     v.    Joiner,    522    U.S.   136,       142   (1997),    and    the

Government     did    little,   if     anything,     to   protect   the    generous

ruling it had obtained from the district court from morphing

into error.

                                        III.

     The Government contends that even if it was error for the

district court to admit Agent Dayton’s testimony as it did, the

error was harmless because there was enough evidence outside of

                                         33
Agent Dayton’s testimony to justify the jury’s conviction of

Garcia on all five counts. We disagree with the Government’s

harmlessness metric.

       When      nonconstitutional          error    has   been      established      in   a

criminal case, “the Government must demonstrate that the error

did not have a ‘substantial and injurious effect or influence in

determining the jury's verdict.’”                   United States v. Curbelo, 343

F.3d    273,     278     (4th     Cir.     2003)   (quoting    Kotteakos        v.   United

States,      328      U.S.   750,    776    (1946)).    The    Government       points     to

other evidence presented at trial, relying on the principle that

“where       there      is    a     significant       amount    of     evidence       which

inculpates a defendant independent of the erroneous testimony,

the    error     is    considered        harmless.”    Johnson,      617   F.3d      at   295

(citing United States v. Banks, 482 F.3d 733, 741-42 (4th Cir.

2007)). The Government’s reliance on the dictum from Johnson, in

which       we   refused     to     find    the    error   harmless,       is   seriously

misplaced. 12


       12
        In Johnson, the prosecution persuaded the district court
to admit the decoding expert’s opinion testimony as lay opinion
evidence under Rule 701. We found error in that ruling, 617 F.3d
at 292-93, and we declined to accept the prosecution’s fallback
contention that the testimony could have been admitted as expert
opinion evidence under Rule 702, id. at 294-95, and we declined
to find that the error was harmless. The case at bar is, like
Johnson, but another example of the vagaries in theoretical,
methodological, and foundational challenges such testimony
engenders and why, when not properly managed, there is an
increased likelihood of error, perhaps harmless in some cases,
(Continued)

                                              34
      Indeed, just as in Johnson, the Government’s case here is

not ironclad. The Government first points to the testimony of

Rodriguez,   who    testified    that       he    regularly       sold    heroin    to

Garcia,   interpreted     certain       terms,       and    no     doubt     was    an

impressive   witness.     Rodriguez’s            credibility       was     put     into

question, however, not least because his testimony was in return

for sentencing considerations by the Government in a New York

prosecution in which he faced a maximum potential sentence of

life in prison and, in addition or alternatively, deportation to

the   Dominican    Republic,    perhaps      leaving       behind    his    American

citizen spouse. Of course, the jury was unquestionably entitled

to credit the testimony of Rodriguez, every bit as much as the

Government   was     entitled     to        call     him     as     its     witness.

Nevertheless, Rodriguez’s testimony required searching scrutiny,

for, as the prosecutor explained to the jury in its closing

argument, “when you’re trying the devil, sometimes you’ve got to

go to hell to get your witnesses.” J.A. 1735.

      The Government also points to police surveillance of the

coconspirators,     in   Baltimore      and        elsewhere,       but    save    one



perhaps not in others. Cf. Galloway, --- F.3d at ---, 2014 WL
1424939 at *4-6 (affirming convictions after admission of
decoding expert testimony upon plain error review); United
States v. Hassan, 742 F.3d 104, 135 (4th Cir. 2014) (affirming
after finding proper admission of lay opinion testimony).



                                       35
instance when Garcia was seized while directly involved with

drugs in April 2009, these observations hold no determinative

weight absent the addition of the illuminating wiretap calls and

the coded language referring to drugs. Each of the other counts

for    possession,     and     Garcia’s    involvement         in   the   conspiracy,

rests on the connection between various individuals that was

only established through the hundreds of calls played at trial.

And    while   the    contents     and    context    of   many      of    these    calls

indisputably point to illegal activity, we are unable to hold

that    the    jury    was    unaffected       by   Agent      Dayton’s     unadorned

interpretations.

       These fundamental flaws are exemplified in the very piece

of    evidence   the   Government        points     to   against      Garcia      in   its

harmlessness argument: Feliciano’s call to Garcia after drugs

were seized from her bags at a travel plaza. In testifying about

that call, between Feliciano and Garcia, Agent Dayton opined

that the term “stuff” was “code for heroin.” J.A. 1190. As with

so much of Agent Dayton’s testimony, no foundation was laid, and

there was no explanation for the coding interpretation, leaving

us to speculate that perhaps the basis of her opinion was the

fact that the agents had just seized 500 grams of heroin from

Feliciano.     Without       the   remotest    effort     by    the   Government       to

justify such testimony under Rule 702, Agent Dayton’s “expert

opinion” was little more than an expert veneer glossed on an

                                          36
item of evidence, e.g., the meaning of the term “stuff,” that

the jury was eminently able, if it so chose, to find the meaning

of for itself. But the jury was deprived of an opportunity to

put to use its commonsense, mature judgment by the Government’s

gilding of the expert witness lily.

      Even if there was more uncompromised evidence, as Garcia

points out, this Court’s inquiry is not “‘merely whether there

was   enough   [evidence]   to    support     the   result,   apart      from     the

phase affected by the error. It is rather, even so, whether the

error itself had substantial influence.’” Curbelo, 343 F.3d                        at

286   (quoting   Kotteakos,      328   U.S.   at    765).   Here,   in     a    trial

spanning twelve days, Agent Dayton testified on six different

days, recalled to the stand eighteen times. From the beginning

of the trial to the end of the trial, the calls and the meaning

of the words used in those calls were the centerpiece of the

Government’s case. There was little direct evidence connecting

Garcia to three of the four actual possession charges, and law

enforcement never observed Garcia actually exchanging drugs or

money with any coconspirators. We cannot find Agent Dayton’s

testimony harmless under the circumstances.

                                       IV.

      In Wilson, we criticized defense counsel for failing to

react   aggressively   to     nudge     the    district     court     to       better

exercise its “gatekeeping” responsibilities in respect to a law

                                       37
enforcement witness decoding expert. Wilson, 484 F.3d at 278 n.5

(“Appellants deserve some of the blame for those rare instances

where improper testimony slipped through the gate’s cracks.”).

Here,    no   such   scolding     is    appropriate.         Garcia    timely    and

repeatedly objected regarding the foundational sufficiency and

methodological reliability of the agent’s expert testimony, and

he specifically pointed to the risk of prejudice arising from

the agent’s dual capacity as both an expert and fact witness.

Counsel objected early and often, always respectfully and, on

occasion,     with   success.     But   even       when    the     district     court

sustained     some   objections     (after     a     bench       conference),     the

Government would often “move on” to its next question or its

next area of interest, leaving prejudicial effects hanging in

the air. Although here, as usual, the quantum of prejudice is

not   susceptible    of   exact   measurement,        we   are     persuaded     that

Garcia has established his entitlement to relief.

      For the reasons set forth above, the judgment is vacated

and     the   case   is    remanded      for       further       proceedings      not

inconsistent with this opinion.

                                                           VACATED AND REMANDED




                                        38
