                FOR PUBLICATION

  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT


UNITED STATES OF AMERICA,                 No. 18-10482
                 Plaintiff-Appellee,
                                            D.C. No.
                 v.                      4:17-cr-00894-
                                          JGZ-BGM-1
ENRIQUE VALENCIA-LOPEZ,
             Defendant-Appellant.          OPINION

      Appeal from the United States District Court
               for the District of Arizona
      Jennifer G. Zipps, District Judge, Presiding

         Argued and Submitted March 5, 2020
                  Phoenix, Arizona

                 Filed August 19, 2020

  Before: Michael Daly Hawkins, John B. Owens, and
           Mark J. Bennett, Circuit Judges.

              Opinion by Judge Bennett;
               Dissent by Judge Owens
2            UNITED STATES V. VALENCIA-LOPEZ

                          SUMMARY *


                          Criminal Law

    The panel vacated convictions for four drug felonies in
connection with the defendant’s transportation and
importation of marijuana, and remanded for a new trial, in a
case in which the defendant, who claimed that he acted under
duress, argued that the district court erred in allowing an ICE
supervisory special agent’s testimony.

    The defendant claimed that armed gunmen seized his
truck in Mexico and held him at gunpoint for several hours,
during which time a confederate (or confederates) of the
gunmen drove the truck away and returned; and that the
gunmen told the defendant to continue driving and pretend
nothing had happened or they would kill him and his family.

    The ICE special agent testified as an expert for the
government. A key part of his testimony was that the
likelihood drug trafficking organizations would entrust a
large quantity of illegal drugs to the driver of a commercial
vehicle who was forced or threatened to comply was
“[a]lmost nil, almost none.”

    The panel held that, in allowing the special agent to
testify, the district court abused its discretion by not properly
fulfilling its gatekeeping role under Daubert v. Merrill Dow
Pharmaceuticals, Inc., 509 U.S. 579 (1993), and Kumho Tire
Co. v. Carmichael, 526 U.S. 137 1999), where it made no


    * This summary constitutes no part of the opinion of the court. It
has been prepared by court staff for the convenience of the reader.
            UNITED STATES V. VALENCIA-LOPEZ                 3

reliability findings, as required by Fed. R. Evid. 702, about
the special agent’s testimony, and where the record contains
no evidence as to why the special agent’s experience and
knowledge equals reliability for the “almost nil, almost
none” testimony. The panel explained that dismissing an
argument as “going to the weight, not admissibility” of the
expert’s testimony is not a reliability determination; and
noted that before the special agent said those words on the
witness stand, the government had never told the court or the
defendant that the special agent would so testify. The panel
wrote that even if it were to consider the special agent’s
explanations of his experience after he was qualified as an
expert, such as on cross-examination, that evidence still does
not explain the methodology by which he reliably concluded
that drug trafficking organizations almost never use coerced
drivers. The panel concluded that the error was not
harmless.

    Dissenting, Judge Owens wrote that even if the district
court erred by not explicitly finding reliability (an issue he
doesn’t reach), the record, in light of the special agent’s
background and experience, sufficiently establishes his
testimony was reliable and admissible under Daubert, such
that the “lack of an explicit finding of reliability” was
harmless.
4             UNITED STATES V. VALENCIA-LOPEZ

                             COUNSEL

J. Ryan Moore (argued), Assistant Federal Public Defender;
Jon M. Sands, Federal Public Defender; Office of the
Federal Public Defender, Tucson, Arizona; for Defendant-
Appellant.

Corey J. Mantei (argued), Assistant United States Attorney;
Robert L. Miskell, Chief, Appellate Section; Michael Bailey,
United States Attorney; United States Attorney’s Office,
Tucson, Arizona; for Plaintiff-Appellee.


                              OPINION

BENNETT, Circuit Judge:

    Enrique Valencia-Lopez, a truck driver, was transporting
15,000 kilograms of bell peppers from Mexico to Arizona.
Customs and Border Protection officers stopped him at the
border and found over 6,000 kilograms of marijuana hidden
within the pepper packages. Valencia-Lopez was convicted
of four drug felonies for his transportation and importation
of the marijuana. 1 He was sentenced to 120 months.

    Valencia-Lopez claimed he acted under duress; that
armed gunmen seized his truck in Mexico and held him at
gunpoint for several hours. During that time, a confederate
(or confederates) of the gunmen drove the truck away and
    1
       The jury convicted Valencia-Lopez of: (1) conspiracy to possess
with intent to distribute marijuana, in violation of 21 U.S.C. § 846;
(2) possession with intent to distribute marijuana, in violation of 21
U.S.C. § 841(a)(1), (b)(1)(A)(vii); (3) conspiracy to import marijuana, in
violation of 21 U.S.C. § 963; and (4) importation of marijuana, in
violation of 21 U.S.C. § 952(a) and § 960(a)(1), (b)(1)(G).
            UNITED STATES V. VALENCIA-LOPEZ                   5

returned it. The gunmen then told Valencia-Lopez to
continue driving and pretend nothing had happened, or they
would kill him and his family. During the trial, and over
repeated pretrial and trial objections, U.S. Immigration and
Customs Enforcement (“ICE”) Supervisory Special Agent
Matthew Hall testified as an expert for the government. A
key part of his testimony was that the likelihood drug
trafficking organizations would entrust a large quantity of
illegal drugs to the driver of a commercial vehicle who was
forced or threatened to comply was “[a]lmost nil, almost
none.” If this was believed by the jury, it would have gutted
Valencia Lopez’s duress defense.

    On appeal, Valencia-Lopez argues that the district court
erred in allowing Agent Hall’s testimony. We conclude that,
in allowing Agent Hall to so testify, the district court did not
properly fulfill its gatekeeping role under Daubert v. Merrell
Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), and
Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999). On this
record, the error was not harmless, and Agent Hall’s “almost
nil” expert testimony was not reliable, so we vacate
Valencia-Lopez’s conviction and remand for a new trial.

                               I.

    Valencia-Lopez drove his truck and tractor-trailer into
the United States at the U.S.-Mexico border checkpoint in
Nogales, Arizona. The shipping manifest listed a cargo of
bell peppers weighing 14,969 kilograms. During secondary
inspection, the inspectors also found 6,230 kilograms of
marijuana concealed among the bell peppers. After
Valencia-Lopez was arrested, he told the officers he had
been kidnapped in Mexico and forced to drive his truck into
the United States.
6            UNITED STATES V. VALENCIA-LOPEZ

    Valencia-Lopez was charged with four marijuana related
felonies. Before trial, the government stated it would offer
expert testimony that “drug-trafficking organizations do not
typically use unknowing couriers.” After Valencia-Lopez
moved to preclude that testimony, the government amended
the proposed testimony to include a “risk-management
analysis that the use of threatened couriers would place the
narcotics at a higher risk for seizure than using non-
threatened couriers.”

      Valencia-Lopez then filed a second motion to preclude
expert testimony and requested a Daubert hearing. 2 He
argued a Daubert hearing was necessary because “there
[was] no methodology whatsoever to substantiate Agent
Hall’s proposed testimony,” and Agent Hall lacked relevant
expertise. The district court denied both motions. The court
found the proposed expert testimony was relevant and told
the defense there “certainly . . . can be voir dire of the expert
. . . to assure that he is qualified to testify as to these matters”
at trial, “assuming that the government can lay an
appropriate foundation for the expert’s expertise in relevant
matters.” The court did not discuss reliability.

    At trial, the government presented five witnessesfour
customs officers involved in the inspection or arrest, and
Agent Hall. Before Agent Hall’s testimony, defense counsel
asked the court how to proceed, given that Valencia-Lopez
was still disputing Agent Hall’s expertise and asking if the
defense should voir dire Agent Hall after the government
was “done trying to qualify” Agent Hall. The court told
counsel that “the issue is whether or not the questioning
establishes a foundation and then if you don’t believe it does,

     2
       Valencia-Lopez sought a Daubert hearing after the government
replaced its original proposed expert with Agent Hall.
              UNITED STATES V. VALENCIA-LOPEZ                            7

then you would be able to raise that issue or do additional
voir dire at that time,” confirming that defense counsel
would be able to voir dire then.

    On direct examination Agent Hall testified to his law
enforcement background. He explained that he had been
working for Homeland Security Investigations (HSI), a
branch of ICE, since 2010. Before that he worked as a police
officer with the Swinomish Tribe in Washington in 1999-
2000, and the Tohono O’dham Tribe in Arizona from around
2002 through 2010. From 2007 to 2010 he was on the HSI
task force, assigned from the Tohono O’dham Police
Department. Agent Hall then explained his drug smuggling
training, his undercover work as a police officer and
supervisor, and his subsequent work at HSI “working on
crimes with a nexus to the [Tohono O’dham] Nation.” He
noted, for example, that working undercover gave him
insight into how drug trafficking organizations operate
because he “gather[ed] information straight from the horse’s
mouth” and he successfully went undercover in these
organizations “[i]n several instances, on many different
levels.” As an HSI agent, he obtained a commercial trucking
license from a commercial trucking school while in an
undercover capacity. Agent Hall also generally outlined his
experience with setting up undercover drug trafficking deals
across the border, while noting that he did not operate
undercover in Mexico. 3 After Agent Hall testified that he



    3
       The government’s proffer also included Agent Hall’s CV, which
noted that he personally conducted thousands of interviews and also
reviewed thousands of interviews of “marijuana backpackers, guides,
scouts, drug trafficking coordinators, recruiters, resupply load personnel,
stash house operators, recruiters and street level and wholesale
distributors of illegal drugs.”
8           UNITED STATES V. VALENCIA-LOPEZ

had served as an expert witness in federal drug smuggling
cases, the government moved to qualify him as an expert.

    Valencia-Lopez timely objected and requested to either
explain the basis for the objection or be allowed to voir dire
Agent Hall. The court then asked counsel at sidebar to
explain the objection. Counsel explained she would prefer to
voir dire Agent Hall “to show the Court more reasons why
he doesn’t qualify as an expert in this type of case.” The
court asked to hear argument “since [they] were over here.”
After counsel argued that Agent Hall lacked any experience
directly working with drug cartels in Mexico and had not
adequately explained the basis for his specialized
knowledge, and the government responded, the court ruled
the objection went “more to the weight of the evidence as
opposed to its admissibility” and found “the foundation
ha[d] been established” for Agent Hall to testify as an expert
“as to how drug trafficking organizations operate.”

    Valencia-Lopez again objected and asked to conduct a
Daubert hearing through voir dire. The court overruled the
objection, did not allow voir dire, and found a Daubert
hearing was not required “particularly in light of the issues
that were raised in the Daubert hearing about testing and
such that don’t apply to experts such as Agent Hall.” The
court then told the jury that it “recognize[d] Agent Hall as an
expert on the issue of drug trafficking cartels’ operations and
methodology.”

    Once qualified, Agent Hall testified to the similarities
between a drug trafficking organization and a legal business,
the approximate value of the marijuana found in Valencia-
Lopez’s truck, and the risks to a cartel of using an unwilling
or unknowing driver. Finally, in response to the
government’s question about “the likelihood regarding
whether [drug trafficking organizations] would entrust a
             UNITED STATES V. VALENCIA-LOPEZ                     9

large quantity of illegal drugs to the driver of a commercial
vehicle who’s been forced or threatened to comply,” Agent
Hall replied, “[a]lmost nil . . . . Almost nil, almost none.”

    After the government rested, Valencia-Lopez presented
his duress defense. He testified and called other witnesses,
including two experts. Valencia-Lopez first explained that
his brother had disappeared years before, and his family
found his brother’s remains three months later. Valencia-
Lopez believed the cartels killed his brother. Valencia-Lopez
also testified that several years before being stopped at the
border, he had been shot in the back by cartel members while
returning from a work trip in Mexico. He showed his
gunshot scars to the jury.

    Valencia-Lopez next testified to what happened before
he arrived at the checkpoint. He picked up a load of bell
peppers in Sinaloa, Mexico for delivery to Arizona. About
80 kilometers south of the border, he pulled off for a
bathroom stop where he was accosted by two armed men. At
gunpoint, the men took his wallet with his driver’s license, 4
phone, and keys, and placed Valencia-Lopez face down in
the back of their car. The men “took the truck” and “came
back with it” about ninety minutes later. The men returned
the wallet and phone to Valencia-Lopez and told him to keep
driving north. They also told him to pretend nothing had
happened or they would kill him and his family. Valencia-
Lopez followed these directions because he “had no choice.”
The court gave Valencia-Lopez’s requested jury instruction
on duress.



    4
      Valencia-Lopez testified his driver’s license listed his home
address, where he and his family lived.
10          UNITED STATES V. VALENCIA-LOPEZ

    During closing argument, the government relied on
Agent Hall’s testimony to emphasize not only that “the use
of unwilling, unknowing couriers is not a practical way to
smuggle drugs into the United States,” but also that “there is
almost no use of unwitting, unwilling couriers.”

     The jury convicted Valencia-Lopez as charged.

     We have jurisdiction under 18 U.S.C. § 1291.

                              II.

    “We review the district court’s decision to admit expert
testimony for an abuse of discretion.” United States v.
Ruvalcaba-Garcia, 923 F.3d 1183, 1188 (9th Cir. 2019)
(citation omitted). We first “consider whether the district
court identified the correct legal standard for decision of the
issue before it,” and we then “determine whether the district
court’s findings of fact, and its application of those findings
of fact to the correct legal standard, were illogical,
implausible, or without support in inferences that may be
drawn from facts in the record.” United States v. Hinkson,
585 F.3d 1247, 1251 (9th Cir. 2009) (en banc).

    Valencia-Lopez argues that the district court abused its
discretion by admitting Agent Hall’s testimony without
adequately performing its gatekeeping role under Daubert
and Federal Rule of Evidence 702. Under Rule 702, before
admitting expert testimony, “the district court must perform
a ‘gatekeeping role’ [to] ensur[e] that the testimony is both
‘relevant’ and ‘reliable.’” Ruvalcaba-Garcia, 923 F.3d
at 1188 (citing Daubert, 509 U.S. at 597). This gatekeeping
obligation “applies to all (not just scientific) expert
testimony.” United States v. Hermanek, 289 F.3d 1076, 1093
(9th Cir. 2002) (citing Kumho Tire, 526 U.S. at 147).
Valencia-Lopez does not challenge the district court’s
            UNITED STATES V. VALENCIA-LOPEZ                 11

finding that Agent Hall’s testimony was relevant. We thus
look only to whether the district court appropriately
determined that the testimony was reliable. It did not.

     Daubert discussed several reliability factors, including
testing, peer review and publication, known or potential rate
of error, and general acceptance in the relevant scientific
community. 509 U.S. at 592–94. But “[t]he reliability
inquiry is a ‘flexible one,’” and the district court has “broad
latitude to determine” what factors in Daubert, if any, are
relevant to the reliability determination. Estate of Barabin v.
AstenJohnson, Inc., 740 F.3d 457, 463 (9th Cir. 2014) (en
banc) (quoting Kumho Tire, 526 U.S. at 150, 153). The
district court “also has broad latitude in determining the
appropriate form of the inquiry.” Id. (citing United States v.
Alatorre, 222 F.3d 1098, 1102 (9th Cir. 2000)). But “the trial
court’s broad latitude to make the reliability determination
does not include the discretion to abdicate completely its
responsibility to do so.” Elsayed Mukhtar v. Cal. State.
Univ., Hayward, 299 F.3d 1053, 1064 (9th Cir. 2002),
overruled on other grounds by Barabin, 740 F.3d 457. We
have explained that a district court abdicates its gatekeeping
role, and necessarily abuses its discretion, when it makes no
reliability findings. See Ruvalcaba-Garcia, 923 F.3d
at 1189; Barabin, 740 F.3d at 464.

    Daubert and Kumho Tire may be harder to apply when
the expert testimony is “experience-based” rather than
“science-based.” But any such difficulty cannot simply lead
to a “that goes to weight, not admissibility” default, as here.
Indeed, we see a strong argument that reliability becomes
more, not less, important when the “experience-based”
expert opinion is perhaps not subject to routine testing, error
rate, or peer review type analysis, like science-based expert
testimony. The Supreme Court has made it abundantly clear
12          UNITED STATES V. VALENCIA-LOPEZ

that reliability is the lynchpin—the flexibility afforded to the
gatekeeper goes to how to determine reliability, not whether
to determine reliability:

       [W]here [expert] testimony’s factual basis,
       data, principles, methods, or their application
       are called sufficiently into question, the trial
       judge must determine whether the testimony
       has “a reliable basis in the knowledge and
       experience of [the relevant] discipline.”

           ....

            . . . The objective of [Daubert’s
       gatekeeping requirement] is to ensure the
       reliability and relevancy of expert testimony.
       It is to make certain that an expert, whether
       basing testimony upon professional studies or
       personal experience, employs in the
       courtroom the same level of intellectual rigor
       that characterizes the practice of an expert in
       the relevant field.

Kumho Tire, 526 U.S. at 149, 152 (third alteration in
original) (citations and quotation marks omitted).

    Here, the district court made no reliability findings about
Agent Hall’s testimony. Agent Hall had sufficient
experience and knowledge to qualify as an expert. But the
record contains no evidence as to why that experience, by
itself, equals reliability for his testimony that there was
“[a]lmost nil, almost no[]” likelihood drug cartels would do
what Valencia-Lopez testified happened here—coerce him,
at gunpoint, to carry illegal drugs across the border. At the
final pretrial conference, the district court denied Valencia-
Lopez’s motions in limine after finding that Agent Hall’s
            UNITED STATES V. VALENCIA-LOPEZ                  13

testimony would be relevant “assuming the government can
lay an appropriate foundation for the expert’s expertise in
relevant matters.” Nothing was said about reliability. At
trial, the district court again denied Valencia-Lopez’s
objections after the government qualified Agent Hall as an
expert. The district court explained that “the objections and
points . . . rais[ed] go more to the weight of the evidence as
opposed to its admissibility and I believe that the foundation
has been established, so I will recognize Agent Hall as an
expert as to how drug trafficking organizations operate.” But
dismissing an argument as “going to the weight, not
admissibility, of [the expert’s] testimony” is not a reliability
determination. See Nease v. Ford Motor Co., 848 F.3d 219,
230 (4th Cir. 2017).

    Then, Valencia-Lopez again asked for a Daubert
hearing. The court rejected his request because it did not
“believe that a Daubert hearing is required in these
circumstances, particularly in light of the issues that were
raised in the Daubert hearing about testing and such that
don’t apply to experts such as Agent Hall.” Yet as the
government acknowledges, “Agent Hall’s non-scientific
testimony was subject to the same ‘gatekeeping function’
identified in” Daubert and Kumho Tire. While the district
court here did not have to hold a Daubert hearing, see
Alatorre, 222 F.3d at 1105, it still had an unwavering
gatekeeping obligation to determine the reliability of Agent
Hall’s testimony. For example, in Alatorre, we found the
district court satisfied its gatekeeping obligation, even
though it denied a request for a pretrial Daubert hearing,
because it allowed the defendant to conduct lengthy voir dire
at trial before overruling his relevance and reliability
objections. Id. In contrast, the district court here denied
Valencia-Lopez’s requests for voir dire after the government
moved to qualify Agent Hall as an expert, even though it had
14            UNITED STATES V. VALENCIA-LOPEZ

twice stated defense counsel would have this opportunity in
place of a pretrial Daubert hearing. 5 With no Daubert
hearing or voir dire, “‘some [other] reliability determination
must be apparent from the record’ before we can uphold a
district court’s decision to admit expert testimony.” Elsayed,
299 F.3d at 1066 (quoting United States v. Velarde, 214 F.3d
1204, 1210 (10th Cir. 2000)). We find none in the record;
rather, the district court qualified Agent Hall as an expert
without explicitly finding his proposed testimony reliable. 6
That was an abuse of discretion.

    The district court also did not fulfill its gatekeeping role
for a second, more important reason. The court admitted
Agent Hall’s testimony despite the government establishing
no reliable basis for Agent Hall’s “[a]lmost nil, almost none”
expert testimony. See Hermanek, 289 F.3d at 1093. This is
particularly troubling here. Before Agent Hall said those
words on the witness stand, the government had never told
the court or the defendant that Agent Hall would so testify.

     5
       We need not decide today whether a district court fulfills its
gatekeeping role without either allowing voir dire or conducting a
Daubert hearing. Cf. Barabin, 740 F.3d at 463 (noting that the district
court has “broad latitude in determining the appropriate form of the
inquiry”). We note only that voir dire is a recommended method for the
district court to conduct a reliability determination and discharge its
gatekeeping obligations. See 29 Charles Alan Wright, Arthur R. Miller
& Edward H. Cooper, Federal Practice and Procedure § 6264.3 (2d ed.)
(Aug. 2019 update) (“Before the court rules on whether a witness is
qualified to testify as an expert, the opposing party should be afforded
an opportunity to conduct a voir dire examination of the witness
concerning the witness’s qualifications.”).

     6
      An implicit reliability finding is “not sufficient.” Ruvalcaba-
Garcia, 923 F.3d at 1190. Instead, to “satisfy its ‘gatekeeping duty’
under Daubert, the court must ‘make an explicit reliability finding.’” Id.
(quoting United States v. Jawara, 474 F.3d 565, 583 (9th Cir. 2007)).
             UNITED STATES V. VALENCIA-LOPEZ                     15

See Fed. R. Crim. P. 16(a)(1)(G). It is one thing for a witness
with Agent Hall’s expertise to testify as to the risks to a cartel
of using a coerced courier. But that is a far cry from him
essentially testifying that the cartel never does it. Such
testimony would not be allowed, for example, in a civil case
involving a death allegedly caused by an inoculation without
a rigorous reliability evaluation of the causation opinion.7
We know the nature of such evaluation is not carved in stone,
either for that hypothetical case or this real one. But we also
know that this evaluation is required.

     As we explained in Hermanek, the district court’s
gatekeeper role includes a “preliminary assessment of
whether the reasoning or methodology underlying the
testimony is . . . valid and of whether that reasoning or
methodology properly can be applied to the facts in issue.”
Hermanek, 289 F.3d at 1093 (quoting Daubert, 509 U.S.
at 592–93). This “prerequisite to making the Rule 702
determination that an expert’s methods are reliable” requires
the district court to “assure that the methods are adequately
explained.” Id. at 1094. Agent Hall never explained the
methodology, if any, that he relied on to arrive at the near-
zero probability of drug trafficking organizations using
coerced couriers. Rather, the district court appeared to rely
on Agent Hall’s general qualifications to qualify him as an
expert, contrary to our admonition in Hermanek. See id.
Moreover, the record does not establish the method Agent
Hall employed to arrive at his “almost nil” opinion. Indeed,
it is the government’s burden to “establish the reliability of
the principles and methods employed ‘to draw a conclusion
regarding the particular matter to which the expert testimony

    7
       Imagine there the expert being asked: “Dr. Smith, what is the
likelihood that the inoculation caused Ms. Jones’s death?” Reply:
“Almost nil, almost none.”
16            UNITED STATES V. VALENCIA-LOPEZ

was directly relevant.’” Id. at 1094 (quoting Kumho Tire,
525 U.S. at 154). The government did not do so here.

    The government submitted Agent Hall’s general
qualifications in its proffer. Agent Hall also testified to his
law enforcement expertise, his experience with undercover
work, including obtaining a commercial trucking license,
and his experience with other federal cases in which he was
an expert witness. He also noted that most of his cases arose
in the United States and he did not “operate in an undercover
capacity . . . in Mexico.” His qualifications and experience
are relevant, and indeed necessary. But they cannot establish
the reliability and thus the admissibility of the expert
testimony at issue. See id. at 1093–94. Rather, like the expert
in Hermanek, Agent Hall “failed to explain in any detail the
knowledge, investigatory facts and evidence he was drawing
from,” id. at 1094, to eventually conclude the probability of
coercion by drug trafficking organizations was “almost nil.”
Crucially, he failed to link his general expertise with his
“almost nil” conclusion, and by never explaining how his
expertise lent itself to that conclusion, we cannot sort out
what “reliable principles and methods underlie the particular
conclusions offered.” See id.

    Even if we were to consider Agent Hall’s explanations
of his experience after he was qualified as an expert, such as
on cross-examination, that evidence still does not explain the
methodology by which he reliably concluded that drug
trafficking organizations almost never use coerced drivers. 8
For example, his general testimony about his interviews with

     8
      Ordinarily, “[t]he district court’s ‘gatekeeping function under
Daubert ensures that expert evidence is sufficiently relevant and reliable
when it is submitted to the jury.” Nease, 848 F.3d at 231 (noting that
defense cross-examination after qualification was insufficient).
            UNITED STATES V. VALENCIA-LOPEZ                 17

cartel members in Mexico is too “vague and generalized” to
establish any reliable principles or methods, id., from which
to determine the reliability of his almost nil conclusion. Even
had he testified that he was familiar with the “right” type of
cartels; given his lack of experience within Mexico, and with
no explanation of his methodology, “there is simply too great
an analytical gap between” his experience and his
conclusion. Id. at 1095 (quoting Gen. Elec. Co. v. Joiner,
522 U.S. 136, 146 (1997)); accord United States v. Vera,
770 F.3d 1232, 1243, 1247 (9th Cir. 2014) (general law
enforcement experience in the FBI and gang task force
insufficient to support drug expert testimony not based on
reliable methodology).

    The government argues that “Special Agent Hall’s
opinions were based on his personal experiences and
knowledge from his drug investigations. The defendant’s
demand for additional ‘reliable methods’ and ‘sufficient
facts or data’ cited in Rule 702 are not at issue like [in]
Hermanek because Special Agent Hall fully explained the
background for his opinions.” This response is well off the
mark. The issue is not whether Agent Hall had knowledge
and experience sufficient to allow him to testify as an expert
on the modus operandi of drug cartels. He did. Nor is the
issue whether he had sufficient “background for his
opinions.” Rather, the issue is whether he provided a reliable
basis for his opinion that the likelihood of drug cartels using
coerced couriers is “[a]lmost nil, almost none.” As explained
above, he did not.

    The government also contends that several of our
previous cases compel us to find that there was no error here.
Again, the government’s argument fails. For example, the
government cites United States v. Cordoba, 104 F.3d 225
(9th Cir. 1997), a pre-Kumho Tire case. Cordoba held that
18          UNITED STATES V. VALENCIA-LOPEZ

“Daubert applies only to the admission of scientific
testimony” and is “inapplicable” to non-scientific expert
drug organization modus operandi testimony. 104 F.3d
at 230. Kumho Tire held to the contrary. The government
also cites a pre-Daubert case, United States v. Johnson,
735 F.2d 1200 (9th Cir. 1984). But Johnson simply recites
the unremarkable principle that “government agents . . . may
testify as to the general practices of criminals to establish the
defendants’ modus operandi.” Id. at 1202. That does not
change the analysis here. The government also cites United
States v. Sepulveda-Barraza, 645 F.3d 1066 (9th Cir. 2011)
and United States v. Murillo, 255 F.3d 1169 (9th Cir. 2001)
overruled on other grounds by Muehler v. Mena, 544 U.S.
93 (2005). Sepulveda-Barraza is distinguishable because the
reliability of the expert testimony was not at issue. 645 F.3d
at 1072–73. And in Murillo—where the testimony
concerned unknowing rather than coerced couriers—there
was no indication that the trial court failed to make any
reliability finding, and, unlike here, there was “clear
evidence in the trial transcript that an adequate voir dire was
conducted.” 255 F.3d at 1178.

     We do not question that expert modus operandi
testimony is admissible in drug smuggling cases involving
unknowing or coerced couriers. But the government must
still establish that its expert opinions are reliable under the
standards mandated by Daubert and Kumho Tire. The
government failed to do so here.

                              III.

    Having established the district court abused its discretion
by admitting Agent Hall’s testimony without having
performed its gatekeeping function, we must now determine
whether the inclusion of Agent Hall’s testimony was
harmless error. See Ruvalcaba-Garcia, 923 F.3d at 1190.
            UNITED STATES V. VALENCIA-LOPEZ                  19

The government must show that the error was not
prejudicial, that it did not affect a “substantial right.”
Barabin, 740 F.3d at 464. We begin with the presumption of
prejudice, which the government can rebut by showing “it is
more probable than not that the jury would have reached the
same verdict even if the evidence had not been admitted.”
Id. at 465 (citation omitted).

    The error here was not harmless. As the government
accurately states, “[t]he central issue at trial was whether the
defendant’s transportation of marijuana was under duress,”
and “Agent Hall’s testimony about unwilling couriers ‘went
right to the heart of the defendant’s defense’ that he acted
under duress.” The government argues that “[t]he jury was
free to disbelieve the defendant and infer the opposite of his
testimony to support its verdict.” True. But Agent Hall’s
testimony went “to the heart” of the most important issue in
the case. See Barabin, 740 F.3d at 465 (“Prejudice is at its
apex when the district court erroneously admits evidence
that is critical to the proponent’s case.”). And the
government emphasized that testimony in its closing. While
the government also asserts that “[t]he defendant’s
testimony was unbelievable and inconsistent with the
evidence as a whole,” part of the reason it was arguably
unbelievable was Agent Hall’s opinion, and part of the
“evidence as a whole” was that same opinion. Without Agent
Hall, the government could not attack Valencia-Lopez’s
duress defense.

    As the government acknowledges, “the jury heard
competing expert testimony from Special Agent Hall and
Dr. Chalk,” Valencia-Lopez’s expert on drug trafficking
20             UNITED STATES V. VALENCIA-LOPEZ

organizations, 9 “on the topic of whether drug trafficking
organizations use unwilling couriers[.]” 10 Valencia-Lopez
testified that he owned the truck and he suspected something
illegal was put in it. Thus a core question for the jury was
whether he voluntarily transported the drugs or whether he
acted under legal duress. In other words, did the jury believe
Valencia-Lopez (and his expert), or did the jury believe
Agent Hall? Agent Hall testified that in his expert opinion
the chance that “a large quantity of illegal drugs to the driver
of a commercial vehicle who’s been forced or threatened to
comply” is “[a]lmost nil, almost none.” In so saying, Agent
Hall’s expert opinion directly undercut Valencia-Lopez’s
story and his credibility, as well as Dr. Chalk’s expert
testimony. Cf. United States v. Gutierrez, 995 F.2d 169, 172
(9th Cir. 1993) (noting that the “expert testimony of a law
enforcement officer . . . often carries an aura of special
reliability and trustworthiness” (citation omitted)). As such,
allowing Agent Hall to so testify obviously and substantially
impacted the viability of Valencia-Lopez’s duress defense. 11


     9
      Dr. Chalk testified about the resources cartels have in the regions
where Valencia-Lopez lived and travelled through on his way to the
border, the cartels’ use of coercive techniques and threats to recruit
unwilling participants, and their use of coerced and blind couriers along
with willing participants as part of their overall drug trafficking strategy.
     10
       The government did not challenge the admissibility of Dr. Chalk
before the district court but contends now that “if Special Agent Hall was
unqualified to testify as an expert, Dr. Chalk was not qualified either.”
As it was not challenged below, this argument is waived. Further, we fail
to see how finding both experts unqualified aids the government in
showing this error was harmless.
     11
        Because the district court allowed the duress defense to go to the
jury, it must have found that the evidence Valencia-Lopez presented was
enough to support it. See United States v. Ibarra-Pino, 657 F.3d 1000,
              UNITED STATES V. VALENCIA-LOPEZ                          21

    In sum, the government bears the burden of proving the
error was harmless, and it cannot merely speculate that the
jury both disregarded Agent Hall’s testimony and
disbelieved Valencia-Lopez’s testimony. The government
has not carried its burdenthe error here was not
harmless. 12 We thus vacate Valencia-Lopez’s convictions
and remand for a new trial. 13

    VACATED and REMANDED.

1004 (9th Cir. 2011) (noting that the defendant must make “a prima facie
showing of duress in a pretrial offer of proof, or in evidence presented at
trial” to be entitled to a jury instruction on duress (citation omitted)).

    12
        Our cases make clear that when we cannot determine reliability
from the trial record, remand for a new trial is necessary. Barabin,
740 F.3d at 466; United States v. Christian, 749 F.3d 806, 813–14 (9th
Cir. 2014). But even if our circuit allowed a limited remand when the
appellate court could not determine reliability from the record, it would
not avail the government here. The government had a full opportunity to
present its case as to admissibility and failed to show that Agent Hall’s
key opinion was reliable and thus admissible. As such, had the district
court made an explicit reliability determination on this record, it would
still have abused its discretion by admitting the “[a]lmost nil, almost
none” testimony.

    13
       Because we vacate Valencia-Lopez’s convictions and remand for
a new trial, we do not reach Valencia-Lopez’s other issues except one,
because it may arise again if the case is retried.

     Valencia-Lopez argues that Agent Hall’s “almost nil” testimony
violated Rule 704(b). We find that if Agent’s Hall’s testimony had been
otherwise admissible, its admission would not have violated Rule 704(b).
Rule 704(b) provides, “In a criminal case, an expert witness must not
state an opinion about whether the defendant did or did not have a mental
state or condition that constitutes an element of the crime charged or of
a defense. Those matters are for the trier of fact alone.” Fed. R. Evid.
704(b). That rule was not implicated by Agent Hall’s testimony. In
United States v. Morales, 108 F.3d 1031, 1037 (9th Cir. 1997) (en banc),
22            UNITED STATES V. VALENCIA-LOPEZ

OWENS, Circuit Judge, dissenting:

    I respectfully dissent. Even if the district court erred by
not explicitly finding reliability (an issue which I do not
reach), our inquiry does not end there. We can decide on
appeal whether Agent Hall’s testimony should have been
permitted when “the record is sufficient to determine
whether the expert testimony is relevant and reliable.”
United States v. Ruvalcaba-Garcia, 923 F.3d 1183, 1190
(9th Cir. 2019) (per curiam) (citation and alteration
omitted). “And when the record shows that the expert’s
testimony satisfied the requirements for admission, we may
conclude that the district court’s failure to make an explicit
finding of reliability was harmless.” Id. (citation and
internal quotation marks omitted). Thus, before considering
whether Agent Hall’s testimony prejudiced Valencia-Lopez
to require a new trial, if the record suffices, we should
conduct our own “admissibility analysis and decide whether
the admission itself was erroneous.” Id. (citation omitted).

   In my view, the record here amply establishes that Agent
Hall’s testimony had “a reliable basis in the knowledge and
experience of the relevant discipline.” Id. (citation omitted).
The government therefore satisfied its burden to show

we held that Rule 704(b) prohibits “testimony from which it necessarily
follows, if the testimony is credited, that the defendant did or did not
possess the requisite mens rea.” But we made clear that the only
testimony excluded is “as to a defendant’s actual mental state during the
charged offense or testimony which necessarily would imply that
ultimate conclusion,” and not expert testimony about “facts or opinions
from which the jury could conclude or infer the defendant had [or lacked]
the requisite mental state.” Id. at 1038 (quotation marks and citation
omitted). Agent Hall’s testimony here falls in the latter category.

     We also do not bar the government from seeking to establish the
reliability and admissibility of Agent Hall’s testimony in any new trial.
              UNITED STATES V. VALENCIA-LOPEZ                       23

harmlessness. Id. Agent Hall had almost two decades of law
enforcement experience and was at the time of trial assigned
to supervise the high intensity drug trafficking force in Sell,
Arizona, on the U.S.-Mexico border. Over his career, he had
been involved in 20–30 investigations at the Mariposa port
of entry and worked undercover operations with border-
related crimes and narcotics. This undercover work
involved commercial truck driving and setting up
transportation of narcotics across the border and further into
the United States. His information was sourced from his
undercover work, training, and confidential informants.
This type of background and experience qualified Agent
Hall to testify about how drug trafficking organizations
operate. See United States v. Hankey, 203 F.3d 1160, 1169
(9th Cir. 2000) (stating that the reliability of a law
enforcement officer “depends heavily on the knowledge and
experience of the expert, rather than the methodology or
theory behind it”).

    Agent Hall also had previously been qualified to testify
as an expert seven times in federal court for drug trafficking
cases and one alien smuggling case. This fact alone does not
make him reliable, but he testified extensively on direct
examination about the sources for his opinions, which
included interviewing over 1,000 people involved in
smuggling narcotics, reviewing case files and interviews,
and accessing law enforcement databases. See Ruvalcaba-
Garcia, 923 F.3d at 1191 (citing United States v. Alatorre,
222 F.3d 1098, 1108 (9th Cir. 2000)). 1 He was also

    1
      See also, e.g., United States v. Figueroa-Lopez, 125 F.3d 1241,
1247 (9th Cir. 1997) (holding that a law enforcement officer with
extensive training, over 200 investigations, and background as a special
agent at the DEA was qualified to opine on trade techniques employed
by drug dealers, so admitting him without qualifying him as an expert
24           UNITED STATES V. VALENCIA-LOPEZ

extensively cross-examined about the basis for his opinions,
including his knowledge of the Mariposa port of entry and
his work in Mexico.

    In light of Agent Hall’s background and experience, I
think the record sufficiently establishes his testimony was
reliable and admissible under Daubert. See Id. Therefore,
the “lack of an explicit finding of reliability was harmless,”
so I would affirm. Id. (citation omitted).




was harmless); United States v. Rubi, 775 F. App’x 865, 866 (9th Cir.
2019) (unpublished) (concluding in an unknowing courier case that the
government expert’s reliability was established through testimony
“regarding his extensive background and experience in drug trafficking
cases”).
