                 FOR PUBLICATION
 UNITED STATES COURT OF APPEALS
      FOR THE NINTH CIRCUIT

UNITED STATES OF AMERICA,                  No. 06-10173
                Plaintiff-Appellee,
                                              D.C. No.
               v.
                                          CR-05-00519-1-
OSCAR ARMANDO HERNANDEZ-                        DCB
ACUNA,
                                             OPINION
             Defendant-Appellant.
                                      
       Appeal from the United States District Court
                for the District of Arizona
        David C. Bury, District Judge, Presiding

                   Argued and Submitted
         July 10, 2007—San Francisco, California

                   Filed August 16, 2007

   Before: David R. Thompson, Pamela Ann Rymer, and
            Raymond C. Fisher, Circuit Judges.

                 Opinion by Judge Rymer




                           9985
              UNITED STATES v. HERNANDEZ-ACUNA             9987


                         COUNSEL

Raul A. Miranda, Tucson, Arizona, for defendant-appellant
Oscar Armando Hernandez-Acuna.

George Ferko, Assistant United States Attorney, Christina M.
Cabanillas, Appellate Chief, and Paul K. Charlton, United
States Attorney, Tucson, Arizona, for appellee the United
States.


                          OPINION

RYMER, Circuit Judge:

   Oscar Hernandez-Acuna appeals his conviction and sen-
tence for conspiracy and possession of marijuana with the
intent to distribute. The issue is whether the district court’s
rejection of the magistrate judge’s recommendation to sup-
press evidence without conducting its own evidentiary hear-
ing violated due process. In this case, even if the court should
9988            UNITED STATES v. HERNANDEZ-ACUNA
have held a de novo hearing, the same witnesses who testified
before the magistrate judge at the suppression hearing testi-
fied at trial about why they stopped and searched Hernandez-
Acuna’s car. Hernandez-Acuna asked the court to consider its
ruling again after the close of evidence. In these circum-
stances, we conclude that remand for another hearing is not
required. Accordingly, we affirm.

                                   I

   On the night of February 19, 2005, Border Patrol Agents
John Howard, Luis Santiesteban, and Anthony Ortiz were sta-
tioned in two marked vehicles next to State Road 83 near
Vail, Arizona, on the lookout for smugglers. Howard and San-
tiesteban testified at the suppression hearing and at trial that
they saw a sedan and a pickup truck approaching their posi-
tion; they believed the two vehicles were driving in tandem;
they noticed that both vehicles had temporary registration
tags, which smugglers’ vehicles frequently display; and they
thought the rigid reaction to their floodlights of the driver
(Hernandez-Acuna) and passenger in the sedan was unusual.1
Santiesteban pulled onto the road behind the truck, where he
observed bundles that he believed were marijuana. He radioed
this to Howard, who was following the sedan. Santiesteban
stopped the truck and Howard stopped the sedan. Upon
inquiry they discovered that both vehicles were registered to
the same address and to owners with similar names, and that
both had been purchased from the same dealership within
days of each other. The agents then placed Hernandez-Acuna
under arrest.

   Hernandez-Acuna was indicted in the District of Arizona
on charges of conspiracy and possession with intent to distrib-
ute approximately 302 kg of marijuana in violation of 21
U.S.C. §§ 841 and 846. He filed a motion to suppress evi-
dence seized by the Border Patrol, including in particular the
  1
   Ortiz was indisposed and did not testify on either occasion.
               UNITED STATES v. HERNANDEZ-ACUNA                  9989
registration documents that linked his car to the truck in
which the marijuana was found. The motion was referred to
a magistrate judge, who recommended suppression. He con-
cluded that the government had not set forth an objective and
rational basis to support the conclusion that the sedan and
truck were traveling together.

   The government objected to the Report pursuant to 28
U.S.C. § 636(b)(1), arguing that the magistrate judge’s analy-
sis was inconsistent with United States v. Arvizu, 534 U.S.
266 (2002). The district court disagreed with the magistrate
judge’s report and declined its recommendation. In its view,
under the totality of the circumstances, the agents had a rea-
sonable suspicion that the sedan was traveling in tandem with
the truck.

   Howard and Santiesteban testified during the two-day trial,
as they had at the evidentiary hearing, about the stop and
search of Hernandez-Acuna’s sedan. Defense counsel cross-
examined extensively, and sought to impeach the agents
based in part on the transcript of proceedings before the mag-
istrate judge. At the close of evidence, Hernandez-Acuna
moved for reconsideration of his earlier motion to suppress,
which the district court denied. The jury returned a guilty ver-
dict.

   Hernandez-Acuna timely appeals the district court’s failure
to conduct a de novo evidentiary hearing.2
  2
    Hernandez-Acuna asserts a violation of the Fifth Amendment, not the
Fourth Amendment. The Fourth Amendment is in play only to the extent
that the Fifth Amendment guarantees that the merits of Fourth Amend-
ment challenges will receive an appropriate hearing. Otherwise,
Hernandez-Acuna has waived any claim that the order denying his motion
to suppress was infirm under substantive Fourth Amendment law by fail-
ing to develop any argument with respect to it.
9990          UNITED STATES v. HERNANDEZ-ACUNA
                                II

   We first addressed the question whether a district court
must conduct its own evidentiary hearing before rejecting a
magistrate judge’s recommendation to suppress evidence in
United States v. Bergera, 512 F.2d 391 (9th Cir. 1975). At
issue there was the reach of our decision in Campbell v.
United States District Court for the Northern District of Cali-
fornia, 501 F.2d 196 (9th Cir. 1974), which had upheld the
constitutionality of the Federal Magistrates Act, 28 U.S.C.
§ 636, against an as applied challenge under Article III of the
Constitution. Campbell had established that it is consistent
with Article III for a magistrate judge to conduct a suppres-
sion hearing, provided that the district judge makes the ulti-
mate determination granting or denying the motion. 501 F.2d
at 205. Bergera limited the scope of this rule, holding that it
did not extend to a case in which the magistrate judge recom-
mended granting a motion to suppress and the district court
subsequently denied the motion based on the record of the
suppression hearing conducted by the magistrate judge. 512
F.2d at 392-94.

   [1] Subsequently, the Supreme Court’s decision in United
States v. Raddatz, 447 U.S. 667 (1980), confirmed the correct-
ness of Campbell’s holding that magistrate judges may con-
duct suppression hearings. The Supreme Court recognized
that the issue has a due process dimension in addition to the
Article III concerns that we addressed in Campbell, but it con-
cluded that use of magistrate judges to conduct suppression
hearings comported with the Due Process Clause as well as
with Article III. 447 U.S. at 680, 683-84. However, the Court
clarified that its decision did not reach the issue of the process
required when a district judge rejects a magistrate judge’s
proposed findings on credibility. Id. at 681 n.7. For a district
judge to reject a magistrate judge’s dispositive findings on
credibility without himself hearing the witnesses’ testimony
could, the Court noted, “give rise to serious questions.” Id.
Consequently, we reaffirmed in United States v. Ridgway, 300
              UNITED STATES v. HERNANDEZ-ACUNA               9991
F.3d 1153 (9th Cir. 2002), that a district court errs when it
does not conduct a de novo evidentiary hearing if it rejects the
credibility finding of a magistrate judge who recommends
granting a motion to suppress. Id. at 1156-57. We further clar-
ified that this rule encompasses all credibility findings, with-
out any limitation to findings based on a witness’s demeanor,
id. at 1157, but we did not decide whether the same rule
applies to a reversal based on findings other than credibility,
id. at 1157 n.3.

   [2] Here, the magistrate judge did not make an express
credibility finding, nor, unlike Ridgway, did the district court
expressly reject findings with respect to the agents’ credibil-
ity. However, the magistrate judge’s Report does state that he
“finds it troubling that upon the Court’s open-ended question-
ing, the agents’ responses as to why they believed the vehicles
were connected indicates that the issue of the temporary
plates did not arise until after the agents reviewed the registra-
tion once the Defendant was stopped, i.e., the confirmation
alone that the truck was transporting marijuana precipitated
the stop.” In context, this statement is ambiguous. It could be
taken to implicate the agents’ credibility — or it could be
taken as an “aside” that was unnecessary to the magistrate
judge’s legal judgment that the presence of lawful temporary
plates on two vehicles being driven in a lawful manner is not
a basis for believing that the vehicles are being driven
together. We cannot tell how the district court interpreted the
magistrate judge’s statement, but the court’s order appears to
assume that the agents factored their observation of temporary
license plates into the reasonable suspicion calculus. To the
extent the magistrate judge’s statement was intended as a
reflection on the agents’ credibility, it should not have been
rejected absent live testimony upon which to do so.

   [3] We need not wrestle further with the ambiguity, how-
ever, because in this case if there were error, it was cured by
the instant replay that occurred at trial. As the Supreme Court
indicated in Carroll v. United States, 267 U.S. 132, 162
9992          UNITED STATES v. HERNANDEZ-ACUNA
(1925), and we echoed in United States v. Sanford, 673 F.2d
1070, 1072 (9th Cir. 1982), denial of a motion to suppress
may be sustained on the basis of evidence presented at trial.
The reason is: “[t]he whole matter was gone into at the trial,
so no right of the defendants was infringed.” Carroll, 267
U.S. at 162. Howard and Santiesteban were the only witnesses
at the suppression hearing, and both testified at trial about
when and why they thought Hernandez-Acuna’s sedan was
linked to the truck. In this, Hernandez-Acuna’s situation is
unlike Bergera, where one witness who testified at the sup-
pression hearing did not testify at trial. Howard and Santieste-
ban were examined and cross-examined about all aspects of
the stop, including the temporary tags and what they signified.
Their credibility was attacked. The district judge was able to
see and hear their live testimony and to observe their demea-
nor. He could evaluate their trustworthiness and the weight of
the factors they considered for himself. In short, the judge had
the opportunity to do exactly what he would have done had
he held an evidentiary hearing. Of course it cannot be gain-
said that trials serve a different function from evidentiary
hearings; but judges can, and occasionally do, revisit their
pretrial rulings as a trial runs its course and testimony or
exhibits are adduced that put threshold decisions in a different
light. Moreover, there is no question the parties and the court
understood that suppression was on the table; Hernandez-
Acuna specifically asked for the court to rule again.

  [4] In these circumstances, we conclude that there is no
need for another evidentiary hearing.

  AFFIRMED.
