                 FOR PUBLICATION
 UNITED STATES COURT OF APPEALS
      FOR THE NINTH CIRCUIT

UNITED STATES OF AMERICA,             
               Plaintiff-Appellant,
               v.                           No. 06-30192
W. R. GRACE; ALAN R. STRINGER;
HENRY A. ESCHENBACH; JACK W.                 D.C. No.
                                          CR-05-00007-DWM
WOLTER; J. MCCAIG; ROBERT J.
                                              OPINION
BETTACCHI; O. MARIO FAVORITO;
ROBERT C. WALSH,
            Defendants-Appellees.
                                      
      Appeal from the United States District Court
              for the District of Montana
    Donald W. Molloy, Chief District Judge, Presiding

                  Argued and Submitted
            July 26, 2006—Seattle, Washington

                    Filed July 12, 2007

  Before: J. Clifford Wallace, Kim McLane Wardlaw and
             Raymond C. Fisher, Circuit Judges.

                Opinion by Judge Fisher;
              Concurrence by Judge Wallace




                           8305
                UNITED STATES v. W. R. GRACE               8309


                         COUNSEL

Sue Ellen Woolridge, Assistant Attorney General, William W.
Mercer, United States Attorney, Kris McLean, Assistant
United States Attorney, Todd S. Aagaard, Kevin M. Cassidy,
Allen M. Brabender (argued), Attorneys, United States
Department of Justice, Environmental and Natural Resources
Division, Washington, D.C., for the plaintiff-appellant.

Laurence A Urgenson, William B. Jacobson, Tyler D. Mace,
Michael D. Shumsky (argued), Kirkland & Ellis LLP, Wash-
ington, D.C., for the defendants-appellees.


                          OPINION

FISHER, Circuit Judge:

   This appeal presents us with two questions. First, we must
decide whether the government has adequately complied with
the certification requirements of 18 U.S.C. § 3731, which
gives us jurisdiction to hear this interlocutory appeal. Second,
assuming we have jurisdiction, we must decide whether the
district court exceeded its authority in issuing pretrial orders
8310             UNITED STATES v. W. R. GRACE
that: (1) required the government to submit a pretrial list of
witnesses and later precluded the government from using any
unlisted witnesses in its case-in-chief; and (2) precluded the
government’s identified expert witnesses from relying on doc-
uments not disclosed prior to a disclosure cutoff date. We
hold that the government has now complied with its certifica-
tion requirements, and that in some respects the district
court’s pretrial orders were improper.

   FACTUAL AND PROCEDURAL BACKGROUND

   W.R. Grace & Company mined and processed vermiculite
ore outside of Libby, Montana from the early 1960s until the
early 1990s. In February 2005, the United States obtained a
49-page, 10-count indictment against Grace and several of its
officers, alleging that Grace committed criminal acts related
to improper disposal of asbestos-contaminated vermiculite
spanning 26 years and creating at least 1,200 victims and 230
potential witnesses.

   In March 2005, after considering the parties’ pretrial con-
ference submissions, the district court entered a case manage-
ment order setting a “firm” trial date of September 11, 2006
and establishing various pretrial discovery obligations and
deadlines. Relevant here, the March 2005 order specified an
April 29, 2005 deadline for the government to produce all dis-
coverable materials specified in Rule 16(a) of the Federal
Rules of Criminal Procedure as well as those the government
had identified in an earlier statement of proposed discovery
disclosures; a May 27, 2005 deadline for the government’s
preliminary list of intended witnesses and exhibits; and a Sep-
tember 30, 2005 deadline — almost a year before trial — for
a “finalized list of witnesses and trial exhibits, including final-
ized disclosure of prosecution’s expert witnesses.” The gov-
ernment did not object to the court’s order.

   On September 30, the government notified the district court
that it had given the defendants (hereinafter, collectively
                 UNITED STATES v. W. R. GRACE                8311
“Grace”) “the government’s final witness list and final exhibit
list,” but noted that it would “continue to investigate this case
through close of all evidence at trial” and therefore “reserve[d
the] right to update its witness list and exhibit list through the
close of all evidence at trial.” In its March 2005 submission
to the court, the government had noted the same reservation
of right. The government’s qualified disclosure prompted con-
cerns on Grace’s part (apparently aggravating a history of dis-
covery disputes between the parties), leading the district court
to hold a status conference in early December 2005.

   The court found the government’s argument in support of
its open-ended qualifier to its “final” list troublesome. It noted
that the government’s list of witnesses had gone from an esti-
mated 60 to 80 in March to some 233 in September, and that
the government had initially stated it would have been pre-
pared to try the case in September 2005, some “three months
ago. It cannot now credibly claim that it is necessary to con-
tinue adding witnesses to an already unwieldy list.” Accord-
ingly, the district court issued an order on December 5 (the
“December 2005 order”) limiting the government’s presenta-
tion of witnesses at trial “to those witnesses that have been
disclosed as of the filing of this Order.” Addressing Grace’s
complaints about the adequacy of the government’s expert
witness disclosures, the court also limited the government’s
experts’ reliance on reports and studies to those that “are con-
tained in the discovery produced to date.” On February 17,
2006, the district court denied the government’s motion for
reconsideration but clarified that the government could call
unlisted witnesses and use other evidence if necessary for
rebuttal (the “February 2006 order”).

   The December 2005 and February 2006 orders are the sub-
ject of this interlocutory appeal. The government contends
that the district court erred in limiting the government’s evi-
dence by precluding witnesses and studies it had not disclosed
as of December 5, 2005. Grace primarily argues that we do
not have jurisdiction to hear this interlocutory appeal because
8312                UNITED STATES v. W. R. GRACE
the government has not satisfied its certification burden under
18 U.S.C. § 3731; alternatively, it argues that the district
court’s orders were plainly within the court’s discretion.
When the district court partially dismissed some of the indict-
ment counts after this appeal was filed, see United States v.
W.R. Grace, 429 F. Supp. 2d 1207 (D. Mont. 2006), we
requested that the parties submit supplemental briefs discuss-
ing whether the excluded evidence would be “substantial
proof of a fact material” to the remaining portions of the
indictment for purposes of § 3731.1 Specifically, we asked the
parties to address whether the excluded evidence was such
that “ ‘a reasonable trier of fact could find the evidence per-
suasive in establishing the proposition for which the govern-
ment seeks to admit it[.]’ United States v. Adrian, 978 F.2d
486, 491 (9th Cir. 1992).”

                            DISCUSSION

                           I.   Jurisdiction

   [1] Title 18 U.S.C. § 3731 provides in relevant part:

      An appeal by the United States shall lie to a court of
      appeals from a decision or order of a district court
      suppressing or excluding evidence or requiring the
      return of seized property in a criminal proceeding,
      not made after the defendant has been put in jeop-
      ardy and before the verdict or finding on an indict-
  1
    Based on the five-year statute of limitations applicable to the knowing
endangerment provision of the Clean Air Act, the district court granted
Grace’s motion to dismiss “as to all Defendants with regard to the criminal
conduct charged in Counts II through IV which was complete as of
November 3, 1999.” Grace, 429 F. Supp. 2d at 1245. The government has
filed a separate interlocutory appeal regarding the district court’s partial
dismissal. We do not address the propriety of that dismissal, but because
we conclude that the excluded evidence is material regardless of whether
the dismissal survives appeal, we do not defer resolution of the present
appeal.
                    UNITED STATES v. W. R. GRACE                      8313
      ment or information, if the United States attorney
      certifies to the district court that the appeal is not
      taken for purpose of delay and that the evidence is
      a substantial proof of a fact material in the proceed-
      ing.

(Emphasis added). Section 3731 also states that it “shall be
liberally construed to effectuate its purposes.” We have
explained that the United States’ “right [under § 3731] to
appeal a district court’s order suppressing evidence is condi-
tional.” United States v. Loud Hawk, 628 F.2d 1139, 1150
(9th Cir. 1979) (en banc); see also Adrian, 978 F.2d at 490
(reaffirming Loud Hawk and noting that the government has
to satisfy certain conditions before being entitled to appeal
under § 3731). “First, the appeal is not available if the defen-
dant has been put in jeopardy. Second, the appeal must not be
taken for purpose of delay. Third, the evidence suppressed
must be substantial proof of a fact material in the proceed-
ing.” Loud Hawk, 628 F.2d at 1150.

   [2] The parties agree that the defendants have not been put
in jeopardy and that the appeal is not for the purpose of delay.
The only matter in dispute is whether the excluded evidence
is substantial proof of a material fact. The government insists
that the U.S. Attorney’s mere certification that the evidence
is substantial proof of a material fact is sufficient to establish
our jurisdiction. However, we “require[ ] more than the prose-
cutor’s bare certification that the conditions enumerated in
section 3731 exist.” Adrian, 978 F.2d at 490.2 Rather, “before
we may assume jurisdiction, the government must demon-
strate that the evidence . . . [is] substantial proof of a fact
material to the prosecution.” Id. at 491 (emphasis added); see
  2
    Unlike in Loud Hawk and Adrian, here the district court did not find
that the excluded evidence was immaterial. See Loud Hawk, 628 F.2d at
1150; Adrian, 978 F.2d at 490. There is no suggestion in either case that
in the absence of such a district court finding, the government’s certifica-
tion alone is sufficient.
8314             UNITED STATES v. W. R. GRACE
also id. at 490 (rejecting government’s argument that § 3731
“permits an appeal whenever the government certifies to the
district court that the evidence is ‘substantial proof of a mate-
rial fact,’ regardless of the truth of that certification”); see
also Loud Hawk, 628 F.2d at 1150 (holding § 3731 not satis-
fied as to certain counts notwithstanding “the government’s
protests to the contrary” because substantial proof of material
fact condition had not been met).

   [3] Consequently, in order to satisfy its relatively modest
burden, the government must show that the evidence “makes
a considerable contribution to cognition of a fact relevant to
resolution of the case.” Adrian, 978 F.2d at 491. In determin-
ing whether the government has made the necessary showing:

    we must interpret the [substantial proof] requirement
    . . . in a way which facilitates rather than hinders the
    government’s ability to seek review in this court.
    Thus, we will find the government to have satisfied
    this additional jurisdictional requirement if, assum-
    ing the evidence would be admissible, a reasonable
    trier of fact could find the evidence persuasive in
    establishing the proposition for which the govern-
    ment seeks to admit it.

Id.; see also United States v. Poulsen, 41 F.3d 1330, 1334 (9th
Cir. 1994).

  A. Review Under “Substantial Proof of a Material Fact”
  Standard

   The question of jurisdiction has bedeviled the govern-
ment’s attempted appeal in this case, as the government has
resisted having to go beyond a bare certification to provide
any useful degree of specificity that would help us (and
Grace) evaluate the materiality of the excluded evidence.
Nonetheless, having given the government the chance to sup-
port its certification with substance, we now address its com-
                 UNITED STATES v. W. R. GRACE                8315
pliance with § 3731 in light of the parties’ supplemental
briefing.

   [4] Preliminarily, we first decline Grace’s suggestion that
we dismiss this appeal based on the government’s apparent
reluctance to comply with our certification requirements.
“The failure [timely] to file the certificate does not oust our
jurisdiction but does require us to decide whether to exercise
our discretion under F. R. App. P. 3(a) to dismiss the appeal.”
United States v. Gantt, 194 F.3d 987, 997 (9th Cir. 1999); see
also United States v. Becker, 929 F.2d 442, 444-45 (9th Cir.
1991) (allowing government to file a late § 3731 certificate as
an exercise of discretion). As in Gantt and Becker, we exer-
cise our discretion to excuse the government’s late showing
given that it has now been forthcoming, and there is no indi-
cation that Grace will be prejudiced by our consideration of
the government’s appeal. See Gantt, 194 F.3d at 997
(“[T]hough the government’s oversight has led to a consider-
able waste of judicial resources, the defendant has suffered no
prejudice.”).

   Second, we reject the government’s contention in its sup-
plemental brief that it “need only show that one excluded wit-
ness or one excluded document or study the government’s
experts intend to reply [sic] upon at trial is ‘substantial proof’
of a fact material to the Superseding Indictment.” (Emphasis
in original). The only support the government cites for this
proposition is our earlier statement that the court “must inter-
pret the requirement that the evidence suppressed be ‘substan-
tial proof of a fact material’ in a way which facilitates rather
than hinders the government’s ability to seek review in this
court.” Adrian, 978 F.2d at 491.

   [5] The materiality of one item of evidence does not neces-
sarily justify our jurisdiction to review all of the excluded evi-
dence. The purpose of the materiality requirement is “to force
federal prosecutors to consider whether the appeal is taken for
a proper purpose before they force a defendant to relitigate the
8316               UNITED STATES v. W. R. GRACE
evidentiary issue.” Gantt, 194 F.3d at 997; see also id.
(“Unfortunately, some government attorneys from time to
time treat the § 3731 certification requirement as a mere for-
mality . . . .”). Allowing a single item of evidence to carry the
burden for what could be a wide ranging review of a district
court’s evidentiary rulings would not only undercut the gov-
ernment’s obligation to be selective in its invocation of inter-
locutory appeal, but also invite the appellate court into
refereeing what is in most cases an area of dispute committed
to the discretion of the district court. Thus, we hold that each
type of excluded evidence must meet the “liberally construed”
§ 3731 requirement that it be substantial proof of a material
fact in order to be the subject of a proper interlocutory appeal.

   Third, contrary to Grace’s argument, excluded evidence
need not be “persuasive” as to the ultimate charge but rather
only “persuasive in establishing the proposition for which the
government seeks to admit it.” Adrian, 978 F.2d at 491
(emphasis added). In keeping with the liberal interpretation of
§ 3731, so long as the proposition is relevant to the govern-
ment’s case, that will suffice.

   Finally, we make no determinations regarding admissibility
of any of the apparently excluded evidence, but rather leave
that issue for the district court. See id. (emphasizing that the
court was only “assuming that the evidence would be admissi-
ble” in its § 3731 analysis).

  B.    Excluded Evidence

   [6] The government in its supplemental briefing to us iden-
tifies nine witnesses and three studies that it concludes would
be excluded from its case-in-chief under the district court’s
December 2005 and February 2006 orders and that it argues
meet § 3731’s materiality standard, justifying this interlocu-
tory appeal.3 According to the government, some of this
  3
    The government does not explain why it could not use any of this evi-
dence for rebuttal, as permitted by the February 2006 order. Grace does
not dispute that the evidence would be excluded under both orders, how-
ever.
                   UNITED STATES v. W. R. GRACE             8317
newly disclosed evidence “differ[s] in some extent from the
evidence previously disclosed to [this] Court,” but asserts that
the updating is due to the “United States’s continuous investi-
gation and refinement of its case that is made necessary” by
various circumstances, including the complexity of the case
and evolving medical information about asbestos-related dis-
eases. Grace objects to the government’s “shifting, post-hoc
attempts to justify review . . . [and] its continued failure to
take this Court’s § 3731 jurisprudence seriously.” Nonethe-
less, because the government, albeit belatedly and reluctantly,
has complied with our request to provide more specific infor-
mation, we exercise our discretion to consider the proffered
evidence and the explanation of how its materiality satisfies
the government’s § 3731 certification requirement. See Gantt,
194 F.3d at 997 (opting to consider defective certification
where government corrected its mistake and showed “it does
take the certification requirements of § 3731 seriously”).

  C.   Materiality of the Excluded Evidence

   The government argues that the nine witnesses and three
studies are substantial proof of a fact material to the remain-
ing charges of the indictment. Mindful of § 3731’s mandate
that the certification requirement be construed liberally in
order to allow the government to appeal excluded evidence,
we do not require extensive explanations of how the evidence
fits into the government’s case, but only enough specificity to
evaluate the materiality of the evidence to the issues in the
case. We conclude that the government has now made a
showing sufficient to establish our jurisdiction over this inter-
locutory appeal. “[A] reasonable trier of fact could find the
evidence persuasive in establishing the proposition for which
the government seeks to admit it.” Adrian, 978 F.2d at 491.

  1.   Witnesses

   The government asserts that five of the witnesses will tes-
tify to injuries that will prove that Grace knew of the danger
8318            UNITED STATES v. W. R. GRACE
posed by exposure to asbestos-contaminated vermiculite. We
agree that this testimony is material to the indictment, even as
limited to post-1999 conduct. Even if the testimony relates to
periods for which Grace cannot be held criminally liable
under the statute of limitations, knowledge does not evapo-
rate, and thus the testimony appears relevant. The government
may therefore appeal the exclusion of these witnesses’ testi-
mony.

   Grace does not contest the materiality of the remaining wit-
nesses’ testimony, which relates to Grace’s motive during the
alleged conspiracy and Grace’s alleged obstruction of the
EPA’s investigation.

  2.   Studies Experts Will Rely Upon

   The three apparently excludable documents the government
identifies are studies that it expects various expert witnesses
to rely upon at trial. The government explains that the studies:
(1) show that exposure to lower levels of asbestos-
contaminated vermiculite than were present in the Libby
mines caused asbestos-related diseases; (2) examine the expo-
sure to airborne asbestos fibers in Libby, Montana during var-
ious activities such as sweeping; and (3) differentiate between
those forms of asbestos that are listed as hazardous and those
that are not. We agree that these studies could be substantial
proof of a material fact to the remaining charges.

   [7] In sum, the government has supplemented its initial
conclusory certification with enough specificity to demon-
strate the materiality of the proffered witnesses and studies to
support our jurisdiction under § 3731. We therefore turn to
the question of whether the district court could properly
exclude such evidence.

                         II.   Merits

   We review de novo the scope of the district court’s author-
ity to order discovery under Rule 16 of the Federal Rules of
                 UNITED STATES v. W. R. GRACE                8319
Criminal Procedure. See United States v. Hicks, 103 F.3d 837,
840 (9th Cir. 1996).

  A.   Nonexpert Witnesses

  1.   Authority to Order Disclosure of Nonexpert Witnesses

   The district court’s March 15, 2005 case management order
did not cite any authority for its requirement that the govern-
ment produce a “finalized list of witnesses and trial exhibits”
by September 30, 2005. “As a threshold matter, a court may
not exercise any supervisory power absent a clear basis in fact
and law for doing so.” United States v. Gatto, 763 F.2d 1040,
1046 (9th Cir. 1985) (internal quotation marks and citation
omitted). Presumably, the court was relying on its authority
under Rule 16(d) as well as its authority to manage its docket.
See Atchison, Topeka and Santa Fe Ry. Co. v. Hercules Inc.,
146 F.3d 1071, 1074 (9th Cir. 1998) (holding that “[d]istrict
courts have inherent power to control their dockets” as long
as its exercise is not inconsistent with a rule or statute) (cita-
tion and internal quotation marks omitted).

   [8] Under Rule 16 of the Federal Rules of Criminal Proce-
dure, the government (and the defendant) have certain pretrial
disclosure obligations as to expert witnesses that we shall
address separately, below. With respect to nonexpert wit-
nesses, however, we have explicitly held that a district court
has no Rule 16 authority to order either side to produce a pre-
trial list of expected witnesses, final or otherwise. “A district
court that orders the Government and the defendant to
exchange witness lists and summaries of anticipated witness
testimony in advance of trial has exceeded its authority under
Rule 16 of the Federal Rules of Criminal Procedure and has
committed error.” Hicks, 103 F.3d at 841. Hicks so concluded
after comparing Rule 16’s provisions “permit[ting] the dis-
covery of a party’s intent to rely on expert opinion testimony,
a summary of what the testimony will consist of, and the
bases of the testimony,” with the absence of any provision
8320               UNITED STATES v. W. R. GRACE
permitting the discovery of any other types of witnesses. “The
advisory notes make clear . . . that these subdivisions [govern-
ing discovery of expert witness testimony] do not apply to
witnesses who may offer lay opinion testimony or to sum-
mary witnesses who testify under Federal Rule of Evidence
1006. Of course, these subdivisions also do not, by their
terms, extend to other witnesses that the Government or the
defendant may call at trial.” Id. (internal citation omitted). We
thus reiterated that we cannot “ ‘sanction a practice which
would require a defendant to supply information beyond the
provisions of Rule 16,’ ” and made clear that a district court
that ordered the pretrial disclosure of nonexpert witnesses —
by the defendant or the government — had exceeded its
authority and committed error. Id. (quoting United States v.
Seymour, 576 F.2d 1345, 1349 (9th Cir. 1978)).4

   [9] Although this rule obviously cramps the district court’s
ability to manage a complex case, Hicks’ holding is well sup-
ported by the legislative history of Rule 16. In 1975, the
Supreme Court proposed that Rule 16 be amended to “en-
large[ ] the scope of defendant’s discovery to include . . . a list
of the names and addresses . . . of all witnesses the prosecu-
tion intends to call during its case-in-chief.” See 1975 Enact-
ment, reprinted in Federal Criminal Code and Rules, at 104
(West 2005). The Rules Advisory Committee “endorse[d] the
principle that witness lists are discoverable” and attempted to
“strike a balance between the narrow provisions of existing
law and the broad provisions of the proposed rule” by propos-
ing a slightly different rule that “witness lists need not be
turned over until 3 days before trial” and gave the court the
discretion to “modify the terms of discovery” to require dis-
  4
   Of course, the government may be required to disclose witnesses or
other evidence for other reasons unconnected with a district court’s dis-
covery orders, such as exculpatory evidence as required by Brady v. Mary-
land, 373 U.S. 83 (1963). We express no opinion as to whether the
government here may have violated Grace’s constitutional rights under
Brady and its progeny.
                 UNITED STATES v. W. R. GRACE                8321
closure of the witness lists earlier than 3 days before trial, or
“[to] permit a party not to disclose the identity of a witness
before trial.” Id. at 105. The House version of the bill simi-
larly provided “that each party, the government and the defen-
dant, may discover the names and addresses of the other
party’s witnesses 3 days before trial.” Id. at 107. However, the
“Senate version of the bill eliminate[d] these provisions,
thereby making the names and addresses of a party’s wit-
nesses nondiscoverable. The Conference adopt[ed] the Senate
version.” Id. (emphasis added). As Hicks observed, the Advi-
sory Committee notes reflect the reason the Conference
adopted the Senate’s version of the rule:

    A majority of the Conferees believe it is not in the
    interest of the effective administration of criminal
    justice to require that the government or the defen-
    dant be forced to reveal the names and addresses of
    its witnesses before trial. Discouragement of wit-
    nesses and improper contact directed at influencing
    their testimony, were deemed paramount concerns in
    the formulation of this policy.

Id. at 107; see Hicks, 103 F.3d at 841. Thus the mandatory
regime of witness discovery the district court here attempted
to impose was specifically rejected — after considerable dis-
cussion and over the recommendation of the House and the
Advisory Committee — in the final version of Rule 16. Given
this explicit rejection of requiring pretrial disclosure of wit-
ness lists, the district court cannot rely on its inherent author-
ity to control its own docket to do what Rule 16 does not
permit — no matter how well-intentioned and understandable
in this complex litigation.

   Grace’s suggestion that Hicks is limited to a defendant’s
right not to share witness lists is belied by the foregoing quo-
tations themselves and by the opinion’s careful exploration of
the limitations on discovery embodied in Rule 16. The 1975
Conference Committee notes refer specifically to the Judicial
8322                UNITED STATES v. W. R. GRACE
Conferees’ belief that neither “the government or the defen-
dant [should] be forced to reveal the names and addresses of
its witnesses before trial.” 1975 Enactment, reprinted in Fed-
eral Criminal Code and Rules, at 107 (West 2005) (emphasis
added); see also Hicks, 103 F.3d at 841.

   Grace relies on United States v. Baker, 10 F.3d 1374 (9th
Cir. 1993), for the proposition that in cases of this complexity,
the district court must have the authority to require a binding
pretrial witness list and to exclude any undisclosed witnesses.
In Baker, in the context of a massive “mega-trial,” we noted:

      when the district court requires the prosecution to
      provide a pretrial witness list, it need not allow the
      government, as it did here, to present nearly 100 wit-
      nesses not on that list. . . . While these decisions are
      within the district court’s discretion, we believe that
      stricter adherence to pretrial commitments will guard
      against the prosecution’s underestimating the length
      of its case and will require more thorough and accu-
      rate trial preparation.

Id. at 1392. Baker, however, predated Hicks by several years
and, unlike Hicks, was not addressing the court’s authority in
light of Rule 16 and its legislative history. Rather, the issue
was whether the district court abused its discretion in denying
the defendant’s motion for severance of a trial, and we were
setting forth general policy considerations the district court
should be mindful of when handling large-scale trials. See id.
at 1389. Although the district court in that case had required
the government to produce a witness list, hence the reference
to such a list in the passage quoted above, we explicitly noted
that “the government is not required to furnish the defendant
with a list of witnesses in a non-capital case . . . .” Id. at 1392
n.4.5
  5
   The reference in Baker to the district court’s discretion to exclude
almost 100 witnesses not on a pretrial witness list does suggest that if the
                    UNITED STATES v. W. R. GRACE                         8323
   [10] Accordingly, we conclude that under Hicks and Rule
16, the district court exceeded its authority to the extent that
it ordered the government to disclose a pretrial list of its non-
expert witnesses.

  2. Preclusion of Undisclosed                    Witnesses      from      the
  Government’s Case-in-Chief

   [11] A district court has the power to exclude the govern-
ment’s evidence as a sanction, but only for a “violation of any
constitutional provision, federal statute, specific discovery
order, or any other recognized right.” United States v. Sch-
wartz, 857 F.2d 655, 658 (9th Cir. 1988) (internal quotation
marks omitted); see Gatto, 763 F.2d at 1046 (holding that “the
separation-of-powers principle imposes significant limits” on
the district court’s use of its supervisory powers to exclude
government’s evidence); Fed. R. Crim. P. 16(d)(2)(C). Here,
the district court was legitimately concerned with giving
Grace adequate opportunity and information to prepare its
defense. A “generalized concern” with a defendant’s ability to

government accedes to an order requiring such a list, its agreement may
be enforced by an exclusionary order. Even if so, we do not think the pre-
clusionary orders here have been justified on the record before us. As
noted above, in March 2005, in the notice upon which the district court
based its case management order requiring disclosure of a “final” witness
list, the government specifically stated that it “reserve[d] its right to sup-
plement its witness list . . . through the close of all evidence at trial.” The
government repeated this reservation when making disclosures in compli-
ance with the district court’s order. Thus, at no time did the government
agree to provide an unchangeable witness list.
   Another possibility is that Baker had in mind the district court’s author-
ity to preclude testimony under Federal Rule of Evidence 403 when it
finds the probative value of the evidence is “substantially outweighed by
the danger of unfair prejudice, confusion of the issues, or misleading the
jury, or by considerations of undue delay, waste of time, or needless pre-
sentation of cumulative evidence.” As this option is not before us, we
express no opinion on the propriety of excluding witnesses from the gov-
ernment’s case-in-chief under Rule 403.
8324                UNITED STATES v. W. R. GRACE
prepare is not a sufficient basis upon which to exclude evi-
dence, however. See Schwartz, 857 F.2d at 658.

   [12] It is quite clear that the court was frustrated by the
government’s apparent inability to bring focus and closure to
its case preparation. However, in entering the December 2005
and February 2006 orders limiting the government’s presenta-
tion of nonexpert witnesses in its case-in-chief to those
already disclosed, the district court did not explicitly find the
government to be in violation of any constitutional provision
or federal statute. Because the district court lacked the author-
ity to order the government to disclose its nonexpert witnesses
before trial, the government’s failure to produce a finalized
list of such witnesses by December 5, 2005 cannot serve as
the basis for precluding such undisclosed witnesses from the
government’s case-in-chief.

   Even had the district court found that the government vio-
lated a constitutional provision, a federal statute or a legiti-
mate discovery order, exclusion is not an automatic remedy.
See United States v. Figueroa-Lopez, 125 F.3d 1241, 1247
(9th Cir. 1997) (addressing Rule 16). Exclusion can be “a too
harsh remedy” for discovery violations and generally is “an
appropriate remedy for a discovery rule violation only where
‘the omission was willful and motivated by a desire to obtain
a tactical advantage.’ ” United States v. Finley, 301 F.3d
1000, 1018 (9th Cir. 2002) (quoting Taylor v. Illinois, 484
U.S. 400, 415 (1988)) (emphasis removed).6

   This is certainly so in the case of an order that broadly pre-
cludes evidence, one we would expect to be accompanied by
findings — not present here — of government misconduct
and prejudice to the defendants. See Schwartz, 857 F.2d. at
  6
    We say “generally” because, as the Supreme Court in Taylor empha-
sized, “it is neither necessary nor appropriate for us to attempt to draft a
comprehensive set of standards to guide the exercise of discretion in every
possible case.” 484 U.S. at 414.
                  UNITED STATES v. W. R. GRACE                   8325
659 (noting that the government had not violated any discov-
ery orders or behaved duplicitously); see also United States v.
Gonzales, 164 F.3d 1285, 1292 (10th Cir. 1999) (holding that
“[i]n selecting a proper [discovery] sanction, a court should
typically consider (1) the reasons the government delayed
producing requested materials, including whether the govern-
ment acted in bad faith; (2) the extent of prejudice to defen-
dant as a result of the delay; and (3) the feasibility of curing
the prejudice with a continuance”).

   [13] On the record before us, therefore, we cannot justify
the blanket exclusion from the government’s case-in-chief of
undisclosed nonexpert witnesses as the orders of December
2005 and February 2006 provide. We therefore reverse these
orders insofar as they preclude witnesses the government had
failed to identify as of December 5, 2005.

  B.   Expert Witnesses and Related Documents

  1.   Authority to Order Disclosure

   [14] Although the government cannot be compelled to dis-
close its nonexpert witnesses before trial, Rule 16 establishes
a different regime for expert witnesses and certain categories
of documents and scientific tests. Relevant here, upon a
defendant’s request the government is required to disclose the
expert witnesses it intends to use in its case-in-chief, as well
as a summary of these experts’ expected testimony and the
“bases and reasons for [their] opinions.”7 Fed. R. Crim. P.
16(a)(1)(G); see also 16(a)(1)(E) (documents within the gov-
ernment’s possession, custody or control material to the
defense or that the government intends to use in its case-in-
chief), 16(a)(1)(F) (scientific reports). Once triggered by the
defendant, these discovery obligations are reciprocal. See Fed.
  7
   The 1993 amendments to Rule 16 introduced this provision as Rule
16(a)(1)(E); the 2002 amendments relettered it as Rule 16(a)(1)(G). See
Pub. L. No. 107-273 § 11019, 116 Stat. 1758, 1825.
8326                UNITED STATES v. W. R. GRACE
R. Crim. P. 16(b)(1)(A)-(C). Although Rule 16(a)(1)(G) does
not specify a timeline for such disclosures, the advisory com-
mittee notes state the expectation “that the parties will make
their requests and disclosures in a timely fashion.” Thus,
unlike the disclosure of nonexpert witnesses, once the parties
have triggered Rule 16(a)(1)(G)’s reciprocal disclosure
requirements, the district court has the authority to regulate
such disclosures under Rule 16(d)(2), which gives the court
wide latitude to ensure compliance with Rule 16 discovery
obligations. Because Rule 16(a)(1)(G) specifically refers to
the disclosure of “the bases and reasons” for expert witnesses’
opinions, the district court’s authority also includes the power
to regulate disclosure of documents upon which a party’s
expert witnesses will rely during its case-in-chief. Accord-
ingly, we hold that the district court here acted within its
authority in requiring the government to disclose in advance
of trial a list of its intended expert witnesses and the docu-
ments upon which those expert witnesses would rely.8

  2.    Preclusion of Expert Witnesses

   The district court’s December 2005 and February 2006
orders limiting the government’s case-in-chief to “those wit-
nesses disclosed as of” December 5, 2005 appear to preclude
the addition of expert as well as nonexpert witnesses (other
than as rebuttal witnesses). Rule 16(d)(2) expressly authorizes
as one sanction for noncompliance the exclusion of undis-
closed evidence. Fed. R. Crim. P. 16(d)(2)(C).9 Nonetheless,
   8
     The court’s authority must be tempered on a showing of good cause,
however, by considering the need of experts to update their data before
trial (and where appropriate after trial has commenced) and the need of the
parties to add experts and data to respond to the other parties’ experts.
Moreover, where as here the trial date is postponed, the district court
should also consider the need to extend deadlines.
   9
     “If a party fails to comply with this rule, the court may: (A) order that
party to permit the discovery or inspection; specify its time, place, and
manner; and prescribe other just terms and conditions; (B) grant a continu-
ance; (C) prohibit that party from introducing the undisclosed evidence;
or (D) enter any other order that is just under the circumstances.” Fed. R.
Crim. P. 16(d)(2) (emphasis added).
                 UNITED STATES v. W. R. GRACE                8327
as with the exclusion of nonexpert witnesses, completely pre-
cluding use of an expert witness is an extreme sanction. See
Finley, 301 F.3d at 1018 (addressing the exclusion of an
expert witness’ testimony as a sanction for the defendant’s
alleged failure to give proper notice under Rule 16(b)(1)(C)).
As we held in Finley, “assuming there was an omission of
some sort [with respect to the mandated disclosures regarding
expert witnesses], it was not willfully done to gain a tactical
advantage” and thus the “severe sanction of total exclusion of
the testimony was disproportionate to the alleged harm.” Id.

   [15] Here, in discussing the government’s compliance with
the court’s discovery orders, the district court observed that
the government had failed to comply fully with certain dis-
covery deadlines and that it was the court’s “impression that
the delay is due largely to the government’s practice of adopt-
ing aggressive legal positions in defense of non-disclosure
and waiting for an order of this Court to sort out the dispute.”
As a remedy, it imposed a monitoring requirement on the
government’s compliance. Turning to alleged shortcomings in
the government’s expert disclosures, it required the govern-
ment to supplement those disclosures with specific informa-
tion about the bases for the experts’ testimony. The court did
not, however, clearly find that the government willfully defied
any of its expert disclosure obligations in order to gain a tacti-
cal advantage. The district court thus has not yet articulated
a basis on which it could impose the “severe sanction” of pre-
emptively excluding any additional expert witnesses as part of
the government’s case-in-chief.

   On the other hand, the government has not identified any
expert witness it believes is foreclosed by the court’s Decem-
ber 2005 and February 2006 orders that it wishes to add at this
late date. Should the government seek leave to add such a wit-
ness, we leave it to the district court to address the request in
accordance with the principles we have set forth above.
8328             UNITED STATES v. W. R. GRACE
  3.   Preclusion of New Studies

   The government does express concern that under the
December 2005 and February 2006 orders, its previously
identified experts will not be able to rely on three scientific
studies dealing with asbestos-related exposure levels and dis-
eases because these studies were not disclosed prior to
December 5, 2005. To the extent that the district court’s
orders would preclude these studies because the government
committed a discovery violation by failing to disclose them
earlier, the orders currently lack the proper foundation for
such a sanction. As with the preclusion of expert witnesses,
the district court has not made a finding that the government
willfully failed to disclose these studies earlier in order to gain
a tactical advantage. See Finley, 301 F.3d at 1018. On the
other hand, there is a difference between restricting what an
expert may introduce in support of his testimony and exclud-
ing his testimony altogether, as Finley recognizes. See id. at
1018 (noting that “[t]he severe sanction of total exclusion of
the testimony was disproportionate to the alleged harm”)
(emphasis added). Thus, the district court may be able to
articulate legitimate reasons to restrict the use of the studies
when it addresses the specifics of the government’s failure to
disclose the studies earlier.

   [16] Whether there is any basis for exclusion of these docu-
ments is unclear on the record before us. It appears these
studies were not in existence — at least in their final form —
until after December 2005; all of the studies were “finalized”
or published after the disclosure deadline imposed by the dis-
trict court. Because the government has not yet presented
these studies to the district court, the court has not yet had a
chance to determine whether there is a proper basis for exclu-
sion, or — even if there is such a basis — whether it would
allow the studies’ use as a matter of discretion. We therefore
remand the question to the district court for resolution.
                UNITED STATES v. W. R. GRACE               8329
                      III.   Conclusion

   In sum, the government has now satisfied its burden under
18 U.S.C. § 3731 to certify the materiality of the apparently
excluded evidence, thereby justifying our jurisdiction over
this interlocutory appeal. The district court exceeded its
authority by ordering the government to produce a pretrial list
of nonexpert witnesses, but acted within its authority by set-
ting deadlines for the disclosure of expert witnesses the gov-
ernment intended to present and studies the government’s
expert witnesses intended to rely upon during the govern-
ment’s case-in-chief. Before the district court can exclude a
witness’ testimony for a pretrial disclosure violation — either
directly or by precluding the evidentiary basis for the witness’
testimony — the court must find that the omission was willful
and motivated by a desire to obtain a tactical advantage.

   We are sympathetic to the district court’s attempts to man-
age this large and complex criminal trial. We therefore
emphasize that we do not decide whether circumstances
beyond those we have addressed here may develop that would
justify excluding or limiting testimony or documents — either
as a sanction or as an evidentiary matter. We remand to the
district court for proceedings not inconsistent with this opin-
ion.

  REVERSED and REMANDED.



WALLACE, Circuit Judge, concurring:

   I agree that the government has complied with its certifica-
tion requirements under our present case law, and agree fully
with the disposition. I write separately because I believe that
our cases relating to the government’s certification require-
ments under 18 U.S.C. § 3731 were wrongly decided and
should be reviewed en banc.
8330             UNITED STATES v. W. R. GRACE
   To establish appellate jurisdiction, our case law unaccount-
ably “require[s] more than the prosecutor’s bare certification
that the conditions enumerated in section 3731 exist.” United
States v. Adrian, 978 F.2d 486, 490 (9th Cir. 1992). The stan-
dard to be met by the government is as follows: “assuming
that the evidence would be admissible, a reasonable trier of
fact could find the evidence persuasive in establishing the
proposition for which the government seeks to admit it.” Id.
at 491. This requirement is directly contrary to the language
and aims of the statute and conflicts with the decisions of
every other circuit to have expressed an opinion on the issue.

  The relevant part of section 3731 states:

    An appeal by the United States shall lie to a court of
    appeals from a decision or order of a district court
    suppressing or excluding evidence or requiring the
    return of seized property in a criminal proceeding,
    not made after the defendant has been put in jeop-
    ardy and before the verdict or finding on an indict-
    ment or information, if the United States attorney
    certifies to the district court that the appeal is not
    taken for purpose of delay and that the evidence is
    a substantial proof of a fact material in the proceed-
    ing.

18 U.S.C. § 3731 (emphasis added). The statute also tells us
that its provisions “shall be liberally construed to effectuate
its purposes.” Id.

   As a starting point, a plain reading of the statute indicates
that, contrary to our cases, mere executive certification is suf-
ficient for appellate jurisdiction. As the Eighth Circuit has
observed, “[t]he second and third requirements . . . share the
common modifying phrase, ‘if the United States attorney cer-
tifies to the district court.’ The syntax implies that executive
certification establishes both lack of delay and materiality.”
United States v. Johnson, 228 F.3d 920, 923-24 (8th Cir.
                UNITED STATES v. W. R. GRACE               8331
2000) (citation omitted). “The United States Attorney’s word
is enough; the reviewing court does not consider the truth of
the certification.” Gov’t of the Virgin Islands v. Hodge, 359
F.3d 312, 325 (3d Cir. 2004). Any other reading of the statute
is implausible. Nowhere in the text of the statute is there men-
tion of a requirement of anything more than the prosecutor’s
certification. Indeed, the imposition of such a requirement
violates the admonition that the provisions of the statute be
liberally construed.

   The circumstances surrounding the enactment of the cur-
rent form of section 3731 further demonstrate the error of our
case law. Prior to the 1968 amendments to the statute, the
government’s ability to appeal pretrial decisions in criminal
cases was far more limited. In Di Bella v. United States, the
Supreme Court held that the government could not appeal
from a post-indictment order suppressing evidence. 369 U.S.
121, 131 (1962). The Court stated that allowance of such a
right of appeal “must be sought from Congress and not this
Court.” Id. (citation omitted).

   The problem was clear. If the district court improperly sup-
pressed evidence, the right of the government to appeal was
frustrated. If the defendant were convicted, the government
had no need to appeal. If the defendant were acquitted, double
jeopardy prevented review. Thus, district court suppression
orders were essentially unreviewable by government request.

   After Di Bella, Congress amended section 3731 to allow
for appeals from orders granting motions to suppress evi-
dence. Pub. L. 90-351, 82 stat. 197 (1968). Thereafter, in
1971, Congress expanded the government’s right of appeal to
include all orders suppressing or excluding evidence in a
criminal proceeding, and added the provision requiring that
the statute be “liberally construed.” Pub. L. 91-644, 84 Stat.
1890. The Senate Report on the amendment explains its pur-
pose:
8332            UNITED STATES v. W. R. GRACE
    The amended Criminal Appeal Act is intended to be
    liberally construed so as to effectuate its purpose of
    permitting the Government to appeal . . . from all
    suppressions and exclusions of evidence in criminal
    proceedings, except those ordered during trial of an
    indictment or information. S. 3132 (the 1971 amend-
    ment) places on the face of section 3731 an explicit
    expression of this intent, in view of the restrictive
    judicial interpretations of congressional intent which
    have resulted from the histories of the earlier ver-
    sions of section 3731 despite strong indications in
    the debate on the 1907 act that it should be broadly
    interpreted.

S. Rep. No. 91-1296, 91st Cong., 2d Sess. 18-19 (1970); see
also In the Matter of Grand Jury Empanelled, 597 F.2d 851,
855-56 (3d Cir. 1979) (detailing history of section 3731).

   The Supreme Court has also interpreted the statute and its
history as granting the government a wide power of appeal.
“[T]he legislative history [of the statute] makes it clear that
Congress intended to remove all statutory barriers to Govern-
ment appeals and to allow appeals whenever the Constitution
would permit.” United States v. Wilson, 420 U.S. 332, 337
(1975) (emphasis added). Wilson, which discusses the legisla-
tive history of section 3731 in some detail, goes so far as to
conclude that “it seems inescapable that Congress was deter-
mined to avoid creating nonconstitutional bars to the Govern-
ment’s right to appeal. The District Court’s order . . . is
therefore appealable unless the appeal is barred by the Con-
stitution.” Id. at 339 (emphasis added).

   The statute expresses Congress’s intent to permit all consti-
tutional government appeals from pretrial evidentiary rulings.
See Arizona v. Manypenny, 451 U.S. 232, 243 n.18 (1981)
(“By enacting the current version of 18 U.S.C. § 3731, Con-
gress manifested an intent to remove all statutory barriers to
a criminal appeal taken by the Federal Government” (citations
                 UNITED STATES v. W. R. GRACE               8333
omitted)). There is not the slightest indication that the barrier
created by our circuit is constitutionally based. Instead, we
have created an unwarranted hurdle in violation of the lan-
guage of the statute, its legislative history, and the specific
dictate of the Supreme Court. Although I was on the en banc
court in United States v. Loud Hawk, 628 F.2d 1139, 1150
(9th Cir. 1979) (en banc), and on the panel in Adrian, I am
now convinced they were wrong on this issue. These cases
should be reconsidered by our court en banc, and our ill-
conceived limitations should be abandoned.
