FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

UNITED STATES OF AMERICA,                      No. 07-10553
                Plaintiff-Appellee,
               v.                                 D.C. No.
                                               CR-05-01969-CKJ
JOSE CARDENAS-MENDOZA,
                                                  OPINION
             Defendant-Appellant.
                                          
        Appeal from the United States District Court
                 for the District of Arizona
        Cindy K. Jorgenson, District Judge, Presiding

                 Argued and Submitted
       December 10, 2008—San Francisco, California

                      Filed August 26, 2009

       Before: Sidney R. Thomas and Richard A. Paez,
          Circuit Judges, and Vaughn R. Walker,*
                    Chief District Judge.

           Opinion by Chief District Judge Walker




  *The Honorable Vaughn R. Walker, Chief Judge, United States District
Court for the Northern District of California, sitting by designation.

                               11801
             UNITED STATES v. CARDENAS-MENDOZA           11805
                         COUNSEL

Francisco León, Francisco León P.C., Tuscon, Arizona, for
the appellant.

Diane J. Humetewa, United States Attorney, District of Ari-
zona, Christina M. Cabanillas, Appellate Chief and Bruce M.
Ferg, Assistant United States Attorney, Tuscon, Arizona, for
the appellee.


                         OPINION

WALKER, Chief District Judge:

   Jose Cardenas-Mendoza appeals from a conviction in the
United States District Court for the District of Arizona for
possessing with intent to distribute methamphetamine and
importing methamphetamine. He argues the district court
erred: (1) when it denied a motion for a mistrial following the
government’s reference to prior bad acts in its opening state-
ment; (2) when it denied his request to produce the govern-
ment’s Treasury Enforcement Communication System
(TECS) hit notice; and (3) when it denied his request to strike
the testimony of a government agent under the Jencks Act, 18
U.S.C. § 3500(d), when the government could not produce a
transcript of the agent’s grand jury testimony. We have juris-
diction pursuant to 28 U.S.C. § 1291.

   The district court did not err with respect to the first two
issues on appeal but did abuse its discretion under the Jencks
Act when it did not strike the testimony of a government
agent whose grand jury testimony transcript could not be pro-
duced. Because the error was harmless, however, we affirm
Cardenas-Mendoza’s conviction and sentence.

                               I

  Cardenas-Mendoza was arrested on September 22, 2005 as
he attempted to cross the United States-Mexico border at the
11806        UNITED STATES v. CARDENAS-MENDOZA
DeConcini Port of Entry in Nogales, Arizona. Cardenas-
Mendoza was referred to the secondary inspection area
because he was shaking abnormally and because of a TECS
hit notice on Cardenas-Mendoza’s pick-up truck. During the
secondary inspection, officers discovered a trap door conceal-
ing a secret compartment in Cardenas-Mendoza’s vehicle.
The compartment held thirteen packages of methamphetamine
weighing 8.36 gross kilograms and containing 5.8 kilograms
of 88 percent pure methamphetamine.

   While officers searched his car, Customs and Border Patrol
(CBP) Officer Stonie Costa took Cardenas-Mendoza inside so
that Cardenas-Mendoza could not see the search. Once offi-
cers found the drugs in the truck, Immigration and Customs
Enforcement (ICE) Agent Steve Huerta was summoned to
question Cardenas-Mendoza. Because Officer Costa speaks
Spanish more fluently than Agent Huerta, Officer Costa
assisted with the questioning.

   Agent Huerta and Officer Costa offer one account of
Cardenas-Mendoza’s questioning. They testified at trial that
they began by stating they knew why Cardenas-Mendoza was
there, that Cardenas-Mendoza knew why he was there, and
that they had received information about Cardenas-Mendoza’s
border crossing. In their account, Cardenas-Mendoza
responded by asking “what drugs?” Agent Huerta and Officer
Costa stated that they told Cardenas-Mendoza they had not
mentioned drugs at all.

   Cardenas-Mendoza testified at trial and offered a somewhat
different account of the questioning. According to Cardenas-
Mendoza, Agent Huerta mentioned the drugs in the vehicle
before giving Cardenas-Mendoza his Miranda warnings and
well before Cardenas-Mendoza used the word “drugs.”
Cardenas-Mendoza’s defense at trial was that he was nothing
more than an unknowing dupe of drug smugglers. He testified
that he had lent his truck to his acquaintance “Paco” that
morning with the understanding that Paco return the truck by
             UNITED STATES v. CARDENAS-MENDOZA            11807
3 PM so that Cardenas-Mendoza could cross the border in
time to buy a washing machine.

   Cardenas-Mendoza was indicted on October 19, 2005 on
two counts: for knowingly and intentionally possessing
methamphetamine with intent to distribute in violation of 21
U.S.C. §§ 841(a)(1) and 841(b)(1)(A)(viii) and for importing
the drug into the United States in violation of 21 U.S.C.
§§ 952(a), 960(a)(1) and 960(b)(1)(H). Agent Huerta testified
before the grand jury that same day. On December 21, 2005,
the government received notice that the grand jury reporter,
Ron Lunsford of Arizona Court Reporting, had died. Lunsford
had been responsible for securing his own notes, tapes and
transcripts. While the government recovered most of Luns-
ford’s materials, the government could not locate materials
from Cardenas-Mendoza’s grand jury hearing.

   Prior to trial, the government notified both Cardenas-
Mendoza and the district court that because of Lunsford’s
death, it could not provide the grand jury transcript. Cardenas-
Mendoza moved under the Jencks Act to preclude Agent
Huerta from testifying at trial because Agent Huerta’s grand
jury testimony was not available for cross examination. The
government opposed the motion, arguing it was not responsi-
ble for the missing transcript and recounting the efforts it
undertook to recover the grand jury testimony. The district
court denied the Jencks Act motion, finding that sanctions
were unnecessary in light of the government’s good faith
attempt to provide the transcript. Agent Huerta testified dur-
ing trial, and the defense cross-examined him using notes
Agent Huerta had written contemporaneous to questioning
Cardenas-Mendoza at the border and a report Agent Huerta
had made a few weeks later.

   Cardenas-Mendoza filed an additional motion to compel
disclosure of the Treasury Enforcement Communication Sys-
tem (TECS) hit notice, because he wished to use the docu-
ment to cross examine the agents who interrogated him. The
11808        UNITED STATES v. CARDENAS-MENDOZA
government opposed the motion and provided a copy of the
notice for the district court to review in camera. After deter-
mining that the information contained in the TECS notice was
collateral and that disclosure was not required, the district
court denied the motion.

   The trial began on January 9, 2007. During its opening
statement, the government referred to anticipated testimony
regarding the truck’s hidden compartment and the labor
required to build it. Specifically, the government stated:

    And Officer Bavaro will tell you about this particular
    compartment in the defendant’s truck, how it
    appeared to have been used more than once. In fact,
    there are layers of adhesive where the metal — the
    metal top of the compartment had been opened and
    closed more than once, as well as some indication of
    very recent use, some fresh duct tape holding the
    metal cover in place.

The government also referred to testimony that “defendant
had owned the truck for just over one year.” Cardenas-
Mendoza moved for a mistrial, arguing that the pre-trial report
did not notice the government’s intent to refer to prior bad
acts. The district court denied the motion for a mistrial but
barred any further mention of the compartment’s prior use.
The court also offered to give a curative instruction that the
jury disregard the comments as unsupported by the evidence,
but Cardenas-Mendoza declined for fear the instruction would
only draw more attention to the comment. The government
made no further reference to the compartment’s age or poten-
tial prior use.

   The trial lasted three days. Agent Huerta testified for the
government, and after his direct testimony, Cardenas-
Mendoza renewed his motion to strike the testimony under
the Jencks Act. Cardenas-Mendoza testified on his own behalf
and disputed the government’s claim that he knew drugs were
             UNITED STATES v. CARDENAS-MENDOZA            11809
in the vehicle. Evidence presented by the government
included the testimony of a border agent who observed
Cardenas-Mendoza shaking as he approached the border, an
audio recording of Cardenas-Mendoza speaking with his girl-
friend while he was in jail awaiting trial in which she does not
express surprise that he had been caught at the border, testi-
mony that the secret compartment in the pick-up truck would
have been difficult to construct, and testimony by Agent
Huerta and Officer Costa that Cardenas-Mendoza used the
word “drugs” before they did. The jury found Cardenas-
Mendoza guilty on both counts of the indictment. He was sen-
tenced to two concurrent terms of 200 months imprisonment,
followed by concurrent terms of 60 months supervised
release.

                               II

   We first turn to Cardenas-Mendoza’s claim that the district
court erred when it denied a mistrial in light of the prosecu-
tion’s opening statement. When there are allegations of pro-
secutorial misconduct, the court reviews a district court’s
denial of a mistrial for abuse of discretion. See United States
v. Washington, 

, 1135 (9th Cir. 2006).
Cardenas-Mendoza argues that the government’s reference to
evidence that Cardenas-Mendoza had used the secret com-
partment in his truck before was misconduct that no curative
instruction could fix.

   [1] A district court does not abuse its discretion in denying
a mistrial where the prosecutor’s improper statement is not
prejudicial. 

. The govern-
ment’s inability to produce evidence promised in an opening
statement “would appear to harm the Government’s case
rather than the defense.” United States v. Monks, 

, 955 (9th Cir. 1985) (citation omitted). A curative instruc-
tion may obviate the impact of the government’s statements,
as juries are assumed to follow the court’s instructions. United
11810         UNITED STATES v. CARDENAS-MENDOZA
States v. Steele, 

, 913 (3d Cir. 2002); United
States v. McChristian, 

, 1508 (9th Cir. 1995).

   [2] The district court instructed the jury to base its decision
solely on the evidence presented and reminded the jury that
the attorneys’ statements were not evidence. The district court
also offered a specific curative instruction, which the defense
rejected. While evidence of prior bad acts could have been
prejudicial to Cardenas-Mendoza, the prosecutor’s allusion to
prior bad acts without supporting evidence and coupled with
the court’s instruction is not clearly prejudicial. Thus, the dis-
trict court did not abuse its discretion when it determined that
curative instructions rather than a mistrial were the proper
remedy for the government’s improper opening remarks.

                                III

   Cardenas-Mendoza next argues that the district court erred
when it failed to compel disclosure of the TECS hit notice
under Fed. R. Crim. P. 16 and the Confrontation Clause. The
district court’s discovery rulings are reviewed for abuse of
discretion, United States v. Gonzalez-Rincon, 

,
865 (9th Cir. 1994), while Confrontation Clause challenges
based, as here, on the exclusion of an area of inquiry are
reviewed de novo. United States v. Larson, 

,
1101 (9th Cir. 2007) (en banc).

   [3] Fed. R. Crim. P. 16(a)(1)(E) provides that the govern-
ment must permit the defendant to inspect and copy or photo-
graph items in its possession, custody or control if: “(i) the
item is material to preparing the defense; (ii) the government
intends to use the item in its case-in-chief at trial; or (iii) the
item was obtained from or belongs to defendant.” Because the
TECS notice was never in Cardenas-Mendoza’s possession
and the government did not use the notice itself in its case-in-
chief, Cardenas-Mendoza had to demonstrate that the TECS
hit notice was material to his defense. The district court found
that while the existence of the notice was material, its con-
             UNITED STATES v. CARDENAS-MENDOZA             11811
tents were collateral. The only material use of the TECS
notice was its existence, which did not require disclosure. The
district court did not abuse its discretion in failing to compel
disclosure of the TECS hit notice, as disclosure was not
required under Fed. R. Crim. P. 16.

   [4] Cardenas-Mendoza’s Confrontation Clause argument is
without merit. He argues that the TECS notice would have
assisted him in cross-examining certain government wit-
nesses, but that alone does not amount to a Confrontation
Clause violation. The Confrontation Clause guarantees that an
accused can conduct an effective cross-examination of a wit-
ness offered against him or her. 

.
The Confrontation Clause does not, however, require the gov-
ernment to disclose all documents that might be helpful on
cross-examination. Coleman v. Calderon, 

,
1112 (9th Cir. 1998) rev’d on other grounds sub nom. Calde-
ron v. Coleman, 

(1998). In this case, failure to
disclose what the district court determined to be collateral evi-
dence did not restrict Cardenas-Mendoza’s ability to cross-
examine various government witnesses. Accordingly, we
affirm the district court’s decision not to compel disclosure of
the TECS hit notice.

                               IV

   We next consider whether the district court erred when it
failed to impose sanctions under the Jencks Act, 18 U.S.C.
§ 3500. A district court’s Jencks Act rulings are reviewed for
abuse of discretion. United States v. Alvarez, 

,
1210 (9th Cir. 2004). We review an erroneous decision not to
impose sanctions under the Jencks Act for harmless error.
United States v. Carrasco, 

, 377 (9th Cir. 1976).
For the following reasons, we conclude that the district court
erred when it did not strike the testimony of Agent Huerta, but
we nonetheless affirm because the error was harmless.
11812           UNITED STATES v. CARDENAS-MENDOZA
                                     A

   [5] As an initial matter, the government argues that it was
not obligated to disclose Agent Huerta’s grand jury testimony
because the transcript was never in its possession.1 The gov-
ernment asserts that because the court reporter retained the
tapes at the conclusion of the grand jury proceedings and did
not create a transcript before his death, the government did
not possess the transcript for purposes of the Jencks Act. We
are unpersuaded. “Possession” does not require physical cus-
tody. See United States v. Wallace, 

, 1470 (9th
Cir. 1988) (recognizing that the government possessed wit-
ness’s notes as soon as it realized the witness was using the
notes to testify before a grand jury). Fed. R. Crim. P. 6(e)(1)
requires the government to retain control of grand jury record-
ings unless the court orders otherwise. In this case, the court
reporter had permission to retain physical custody of the tape.
But that permission did not relieve the government of its obli-
gation to maintain the recording. Because Fed. R. Crim. P.
6(e)(1) charges the government with retaining recordings of
grand jury proceedings, it is in possession of the court report-
er’s notes or audio tapes as soon as the recording occurs.

                                     B

   [6] Jencks Act sanctions are triggered when the government
does not provide a witness’s statement properly requested
under the act. United States v. Well, 

, 1384 (9th
Cir. 1978). Ordinarily, sanctions are mandatory, and the dis-
trict court must either strike the testimony of a witness whose
statement is not produced or, alternatively, declare a mistrial.
  1
    Under the Jencks Act, “[a]fter a witness called by the United States has
testified on direct examination, the court shall, on motion of the defendant,
order the United States to produce any statement (as hereinafter defined)
of the witness in the possession of the United States which relates to the
subject matter as to which the witness has testified.” 18 U.S.C. § 3500(b).
The Act specifically includes grand jury testimony within its provisions.
See 18 U.S.C. § 3500(e)(3).
             UNITED STATES v. CARDENAS-MENDOZA             11813
But the district court has some discretion not to impose sanc-
tions in limited circumstances. A district court can decide not
to strike a witness’s testimony where the government acted in
good faith and where a substitute for the missing statement is
available. United States v. Echeverry, 

, 1456
(9th Cir. 1985); United States v. Finnegan, 

, 642
(9th Cir. 1977).

   [7] While the defendant must show she or he is prejudiced
by the missing statement when the government acts in good
faith, see United States v. Angelini, 

, 1308 (9th
Cir. 1979), prejudice exists when no acceptable substitute or
summary of the missing statement is available. 

. Contrary to the district court’s ruling here, the
government’s good faith alone does not excuse production
under the Jencks Act. United States v. Harris, 

,
1253 (9th Cir. 1976); United States v. Riley, 

,
806 (9th Cir. 1999) (affirming denial of sanctions under the
Jencks Act where the government could not produce a state-
ment it had destroyed in good faith and no substitute was
available); 

(“Good faith is the
norm from which neither adverse inferences nor benefits
flow.”).

   [8] The government argues that the district court properly
declined to impose sanctions because the government was not
at fault for the loss of the grand jury testimony. Given that
Fed. R. Crim P. 6(e)(1) specifically entrusts the government
with the responsibility to maintain records of grand jury testi-
mony, however, the government is not accurately depicted as
blameless here. See United States v. Montgomery, 

(5th Cir. 2000) (finding Jencks Act applied where court
reporter, who was in physical possession of grand jury tran-
script, could not be located). Although we have found no
abuse of discretion in a district court’s refusal to impose sanc-
tions for an inadvertent failure to disclose Jencks material, in
these cases some substitute for the material was made avail-
11814        UNITED STATES v. CARDENAS-MENDOZA
able to the defendant. See 

; Fin-

.

   [9] Here, no substitute for the missing grand jury transcript
was available and because of that, we cannot know the con-
tents of Agent Huerta’s testimony. The government argued
that, to the best of its knowledge, the testimony at the grand
jury hearing “was likely not very lengthy,” and that the likely
subjects included the defendant’s contact with the agents at
the border and the discovery of methamphetamine. The gov-
ernment admits, however, that it “cannot confirm that this was
Agent Huerta’s testimony.”

   [10] Agent Huerta testified before the grand jury on Octo-
ber 19, 2005, less than a month after Cardenas-Mendoza was
arrested at the border. His memory of Cardenas-Mendoza’s
arrest was likely fresh enough for him to have testified
beyond the scope of his written report, unlike during trial,
where he relied on his writings to testify. Without guessing
about the content of his grand jury testimony, it is no strain
to conclude that it may have contained an extemporaneous
account of the arrest—an account not available through the
agent’s written notes or report.

   [11] Although the government acted in good faith, no sub-
stitute for the missing statement could be produced. Accord-
ingly, the district court erred when it did not strike Agent
Huerta’s testimony under the Jencks Act.

                               C

  [12] Error alone is not enough to vacate Cardenas-
Mendoza’s conviction—we will reverse only if the error was
not harmless. 

. We have found a
Jencks Act error not to be harmless when the witness’s testi-
mony was key to the prosecution’s case. 

. On the other hand, the error is harmless if the indepen-
dent evidence of guilt is strong enough to sustain the defen-
              UNITED STATES v. CARDENAS-MENDOZA            11815
dant’s conviction beyond a reasonable doubt, 

, and the defendant’s explanation for what hap-
pened is “contradicted by overwhelming evidence.” United
States v. Gracidas-Ulibarry, 

, 1198 (9th Cir.
2000); see also United States v. Smith, 

, 938-39
(9th Cir. 2009).

   [13] Here, Agent Huerta’s testimony played only a small
part in the government’s case. Agent Huerta’s testimony
helped the government establish an element of the prosecu-
tion’s case: that Cardenas-Mendoza knew drugs were in his
vehicle when he crossed the border. But other evidence
offered by the government also demonstrated that Cardenas-
Mendoza knew the drugs were in his vehicle. First, Officer
Costa’s testimony was practically identical to Agent Huerta’s.
While two agents testifying consistently provides stronger
evidence than the testimony of one agent, the testimony of
one agent still carries weight. Furthermore, the government
offered evidence that Cardenas-Mendoza’s girlfriend was not
surprised when she discovered he had been arrested at the
border and evidence that Cardenas-Mendoza was shaking
abnormally as he approached the initial inspection point. Offi-
cer Costa’s testimony combined with independent evidence
that Cardenas-Mendoza knew the drugs were in his vehicle
supports the jury’s verdict.

   [14] The government also offered evidence that Cardenas-
Mendoza’s explanation for the drugs in his vehicle—that he
was the unknowing dupe of drug smugglers—was implausi-
ble. First, a government witness testified that the drug com-
partment in Cardenas-Mendoza’s truck was difficult to
construct, making it unlikely that drug smugglers could have
constructed the compartment in the brief time Cardenas-
Mendoza was not in possession of his vehicle. Furthermore,
Cardenas-Mendoza admitted he did not tell anyone where he
was going in the United States, which would have made
retrieving the drugs difficult for the alleged participants in the
drug smuggling plan of which Cardenas-Mendoza denied all
11816        UNITED STATES v. CARDENAS-MENDOZA
knowledge. Aside from his own testimony, Cardenas-
Mendoza offered no evidence to support his explanation.

   [15] Even if the district court had not erred and Agent
Huerta had not testified, a reasonable jury would have found
Cardenas-Mendoza guilty. Agent Huerta’s testimony was cor-
roborated in full by another officer, and all other evidence
offered by the government supports the jury’s verdict.
Accordingly, the district court’s error in refusing to strike
Agent Huerta’s testimony was harmless.

                              V

   We find the district court erred in refusing to impose sanc-
tions under the Jencks Act. Because the error was harmless,
however, we AFFIRM Cardenas-Mendoza’s conviction and
sentence.