                    FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

UNITED STATES OF AMERICA,                       No. 03-10654
                Plaintiff-Appellee,                D.C. No.
               v.                             CR-02-00062-RCC-
EFRAIN BECERRA-GARCIA,                               BPV
             Defendant-Appellant.
                                                  OPINION

         Appeal from the United States District Court
                  for the District of Arizona
          Raner C. Collins, District Judge, Presiding

                   Argued and Submitted
         October 5, 2004—San Francisco, California

                      Filed February 2, 2005

      Before: Thomas J. Meskill,* Stephen S. Trott, and
          M. Margaret McKeown, Circuit Judges.

                  Opinion by Judge McKeown




   *The Honorable Thomas J. Meskill, Senior United States Circuit Judge
for the Second Circuit Court of Appeals, sitting by designation.

                                1441
              UNITED STATES v. BECERRA-GARCIA          1443


                        COUNSEL

Lee Tucker, Tucson, Arizona, for the defendant-appellant.

Erik J. Markovich, Assistant United States Attorney, Tucson,
Arizona, for the plaintiff-appellee.
1444           UNITED STATES v. BECERRA-GARCIA
                           OPINION

McKEOWN, Circuit Judge

   Efrain Becerra-Garcia challenges the district court’s denial
of his motion to suppress evidence, namely the discovery by
tribal rangers of illegal aliens in his van while crossing the
Tohono O’odham Nation. Although the scope of the rangers’
authority and the location of the stop on reservation land
inform our analysis, this case boils down to a determination
whether the rangers’ stop of Becerra-Garcia was reasonable.
We conclude that it was and thus affirm.

                         BACKGROUND

   The setting of this case is the Tohono O’odham Indian Res-
ervation, which covers a sprawling 90 miles across southern
Arizona. The Tohono O’odham Police Department includes a
division of tribal rangers, officers who have less power than
fully-authorized tribal police officers. The rangers are autho-
rized to patrol the reservation and report suspicious activity to
tribal police officers or the United States Border Patrol.
According to the testimony of two rangers and a tribal police
officer, rangers do not have authority to stop suspicious vehi-
cles. Vehicles that stop voluntarily may be detained until the
arrival of officials who have authority to arrest. If a suspicious
vehicle does not voluntarily stop and instead exits tribal land,
the rangers must let the car go. A ranger may make an arrest
at the direction of a tribal police officer. Thus, the rangers’
primary duties are to patrol, looking for suspicious activity, to
report to the police department and other authorities (usually
the Border Patrol), and to detain suspects who voluntarily
stop.

   Tribal Rangers Andrew Ruiz and Denver Calabaza were
patrolling on a remote dirt road on the reservation when they
saw a van heading north. They were about twenty miles from
the nearest highway and three miles from the nearest village,
               UNITED STATES v. BECERRA-GARCIA               1445
Queens Wells. The tribal police department had, in the pre-
ceding weeks, received complaints of unidentified vehicles
driving through the area. Because trespassing is a significant
problem and only local ranchers typically use the roads in this
vicinity, the rangers make a practice of calling in the license
plate numbers for all unknown vehicles transiting that area.
The rangers did not recognize the van, which did not have a
reservation license plate, and, in keeping with their standard
practice, they followed it in order to report the license plate
number. When the rangers turned on their emergency hazard
lights, the van stopped.

   Almost immediately after the van stopped, the driver,
Becerra-Garcia, got out and walked toward the rangers, leav-
ing the van door open. Both of the rangers had already
stepped out of their jeep. Ranger Calabaza asked Becerra-
Garcia for identification to determine whether he was tres-
passing, but Becerra-Garcia did not speak English. Ranger
Calabaza tried asking in Spanish. In response, Becerra-Garcia
motioned toward the van, and Ranger Calabaza went to the
van to retrieve Becerra-Garcia’s identification. As Ranger
Calabaza approached the van, he saw through the open door
more than twenty undocumented aliens stuffed inside.

   The rangers called the Tohono O’odham Police Depart-
ment, and the police contacted the U.S. Border Patrol. At the
direction of the police department, the rangers detained
Becerra-Garcia and put him in the back seat of their jeep until
the Border Patrol and tribal police officers arrived about thirty
minutes later.

   Becerra-Garcia was later charged with conspiring to trans-
port illegal aliens and with transporting illegal aliens in viola-
tion of 8 U.S.C. § 1324(a)(1)(A)(ii). The district court denied
Becerra-Garcia’s motion to suppress the evidence of the ille-
gal aliens. Becerra-Garcia then entered a conditional plea of
guilty, preserving his right to appeal the denial of his motion
to suppress.
1446           UNITED STATES v. BECERRA-GARCIA
                          DISCUSSION

   We review de novo a district court’s denial of a motion to
suppress. United States v. Garcia, 205 F.3d 1182, 1186 (9th
Cir. 2000). We also review de novo whether an encounter
between a police officer and an individual amounts to a sei-
zure, United States v. Chan-Jimenez, 125 F.3d 1324, 1326
(9th Cir. 1997), and whether an investigatory stop was proper,
United States v. $109,179 in U.S. Currency, 228 F.3d 1080,
1083-84 (9th Cir. 2000).

   At issue in this appeal is the intersection of a tribal policy
and the Fourth Amendment in the context of a motion to sup-
press evidence stemming from a traffic stop. Ironically, the
parties each argue that the Fourth Amendment does not apply.
Even more ironically, both are correct, but not for the reasons
they offer.

   Becerra-Garcia claims that the tribal officers were acting in
a private capacity and therefore state law on citizen’s arrest,
not the Fourth Amendment, comes into play. The Government
counters that Becerra-Garcia stopped voluntarily and thus
there is no Fourth Amendment unreasonable seizure consider-
ation. Both are wrong. The tribal officers were government
agents, not private actors, and the stop was not voluntary. Yet
the Fourth Amendment does not apply because the constitu-
tion does not directly apply to the conduct of tribal govern-
ments. Even so, a federal statute imposes precisely the same
constraints on tribal governments as the Fourth Amendment,
so Fourth Amendment law comes into play.

  I.   THE APPLICABLE LAW

   [1] At the outset, this case presents a thorny issue because
the stop was made by tribal rangers on tribal land, although
the arrest was made by federal officers. The wrinkle is that,
while both parties have briefed this appeal as a Fourth
Amendment case, the Fourth Amendment does not directly
                  UNITED STATES v. BECERRA-GARCIA                       1447
govern the conduct of tribal governments. United States v.
Manuel, 706 F.2d 908, 911 n.3 (9th Cir. 1983) (noting that the
Fourth Amendment does not directly apply to Indian tribes)
(citing United States v. Wheeler, 435 U.S. 313, 329 (1978)).
Nonetheless, the Indian Civil Rights Act (“ICRA”) imposes
an “identical limitation” on tribal government conduct as the
Fourth Amendment.1 Id.; see also United States v. Strong, 778
F.2d 1393, 1397 (9th Cir. 1985). Thus, we analyze the reason-
ableness of the stop under well developed Fourth Amendment
precedent, which nets the same result as an analysis under
ICRA.

   [2] We assume, as have courts before us, that suppression
of evidence in a federal proceeding would be appropriate if
the rangers’ conduct violated ICRA. See United States v.
Male Juvenile, 280 F.3d 1008, 1023 (9th Cir. 2002) (consider-
ing suppression of evidence based on argument that confes-
sion was wrongfully obtained by tribal investigators); Strong,
778 F.2d at 1396-97 (considering suppression of evidence
based on argument that search violated ICRA); Manuel, 706
F.2d at 911-12 (considering suppression of evidence based on
argument that arrest by tribal officers violated ICRA).2

   Becerra-Garcia would have us avoid analyzing this appeal
through the Fourth Amendment lens, not because the rangers
are tribal officers, but because they acted in their capacity as
  1
     The Indian Civil Rights Act (ICRA) provides that “No Indian tribe in
exercising powers of self-government shall . . . violate the right of the peo-
ple to be secure in their persons, houses, papers, and effects against unrea-
sonable search and seizures.” 25 U.S.C. § 1302(2).
   2
     We acknowledge that in the civil context, habeas corpus relief gener-
ally is the sole federal remedy for a violation of ICRA. See Santa Clara
Pueblo v. Martinez, 436 U.S. 49, 69-70 (1978); R.J. Williams Co. v. Fort
Belknap Hous. Auth., 719 F.2d 979, 981 (9th Cir. 1983). But this case does
not rest on a claim under ICRA, only on the application of the “Fourth
Amendment-like” standard imposed by ICRA. In the criminal context,
suppression is a rule of federal criminal procedure rather than a civil rem-
edy for an ICRA violation.
1448           UNITED STATES v. BECERRA-GARCIA
private citizens. Becerra-Garcia suggests that, because the
tribal rangers lack the authority to stop cars, they were not
acting in their capacity as government actors. Instead, he
argues that citizen’s arrest law, not the Fourth Amendment,
controls the analysis. See State v. Chavez, 96 P.3d 1093, 1094
& n.1 (Az. Ct. App. 2004) (applying citizens’ arrest frame-
work to a stop conducted by tribal rangers for the San Xavier
Indian Reservation). He further contends that there is no state
law basis for a citizen’s arrest in these circumstances.

   [3] This approach, while creative, misses the mark. For
Fourth Amendment purposes, an individual is a government
agent if the government knew of and acquiesced in the offi-
cer’s activities, and the party performing a seizure intended to
assist law enforcement and did not act to further his own ends.
United States v. Reed, 15 F.3d 928, 931 (9th Cir. 1994) (citing
United States v. Miller, 688 F.2d 652, 657 (9th Cir. 1982)).
Here, two different government entities — the Tohono
O’odham Police Department and the Border Patrol —
acknowledged and endorsed the patrol activities of the rang-
ers. Indeed, the function of the rangers is to assist the tribal
police department and Border Patrol by policing the remote
corners of the reservation. The rangers stopped Becerra-
Garcia to enforce the criminal trespass laws of the tribal
nation, not to effect a personal benefit. Thus, because ICRA
and the Fourth Amendment impose equivalent restrictions and
because the rangers are government actors, we apply Fourth
Amendment law to resolve this appeal.

  II. THE STOP—A SEIZURE UNDER FOURTH AMENDMENT
  STANDARDS

   The threshold question is whether there was a stop by the
rangers or whether Becerra-Garcia voluntarily stopped his
van. The government takes the position that the Fourth
Amendment is not implicated because Becerra-Garcia stopped
his van voluntarily, not as a result of government intrusion.
               UNITED STATES v. BECERRA-GARCIA              1449
   Becerra-Garcia testified that he stopped because the rangers
flashed their emergency lights whereas the rangers claimed
that they activated their lights only after the van had stopped.
The district court weighed this conflicting testimony and
sided with Becerra-Garcia, finding that the rangers effected a
stop of the van.

   We review the district court’s factual findings for clear
error, and we do not disturb those findings unless “they are
without foundation.” United States v. Diaz-Cardenas, 351
F.3d 404, 407 (9th Cir. 2003). We are especially reluctant to
set aside a determination, such as this one, that depends
wholly on a credibility finding. United States v. Haswood,
350 F.3d 1024, 1028 (9th Cir. 2003) (appellate court gives
“special deference to the district court’s credibility determina-
tions”). Because the district court’s conclusions are well
grounded in the record, we accept the finding that the rangers
stopped Becerra-Garcia. The stop falls squarely within the
purview of the Fourth Amendment. See United States v. Diaz-
Juarez, 299 F.3d 1138, 1141 (9th Cir. 2002) (stating that “the
Fourth Amendment’s prohibition of unreasonable searches
and seizures extends to [an] . . . investigatory stop of a vehi-
cle”). Having determined that a stop occurred, we next ana-
lyze the reasonableness of that stop.

  III. REASONABLENESS OF THE STOP UNDER FOURTH
  AMENDMENT STANDARDS

   The district court held that the rangers’ suspicion of tres-
passing upon the Indian Nation justified the stop. Following
the stop, the rangers detained Becerra-Garcia, conduct that
fell squarely within their authority, and the Border Patrol
made the actual arrest. Becerra-Garcia does not contest the
finding with respect to reasonable suspicion. The sole argu-
ment he offers for why the stop was unreasonable is that the
rangers lacked authority under tribal law to effectuate the
stop.
1450            UNITED STATES v. BECERRA-GARCIA
   Before we address Becerra-Garcia’s argument, we take a
detour to examine the authority of the Tohono O’odham rang-
ers. Becerra-Garcia has not offered proof of any statute or
regulation that circumscribes the rangers’ powers. Instead,
Becerra-Garcia relies entirely on testimony that the rangers
had been told they lacked authority to stop vehicles. The
source of this directive—whether statute, rule, fiat, or policy
—was never explored. Despite the murkiness surrounding the
scope and source of the rangers’ powers, the evidence that the
rangers lacked authority to stop drivers was uncontested. Con-
sequently, for the purposes of this opinion, we assume that
although the rangers had authority to detain individuals on the
reservation, they were not authorized to stop Becerra-Garcia.
We also consider the source of the limits on the rangers’
authority—whatever it may be—to be the equivalent of tribal
law. But, as we discuss below, the rangers’ authority under
tribal law is not the linchpin for determining the admissibility
of the evidence obtained as a result of the stop.

    A.    FRAMEWORK FOR ANALYSIS—FEDERAL LAW GOVERNS
          THE REASONABLENESS OF THE SEIZURE


   [4] We start with the proposition that “[t]he general rule . . .
is that evidence will only be excluded in federal court when
it violates federal protections, such as those contained in the
Fourth Amendment, and not in cases where it is tainted solely
under state law.” United States v. Cormier, 220 F.3d 1103,
1111 (9th Cir. 2000). We have extended this principle to the
context of tribal law, holding that the admissibility of evi-
dence in federal court is determined without regard to tribal
law. Male Juvenile, 280 F.3d at 1023 (“[W]e reject the con-
tention that tribal law should govern the admissibility of state-
ments in federal court. Federal law governs federal
proceedings.”); see also United States v. Hornbeck, 118 F.3d
615, 617 (8th Cir. 1997) (“Federal, not tribal or state, law
governs the admissibility of evidence” in the district court).
                  UNITED STATES v. BECERRA-GARCIA                    1451
   [5] For many years, we left unresolved the question
whether state or federal law governed the reasonableness of
seizures, see, e.g., United States v. Clawson, 831 F.2d 909,
913 (9th Cir. 1987) (holding that the stop of a car was permis-
sible regardless of whether state or federal law applied);
United States v. Contreras-Diaz, 575 F.2d 740, 744 (9th Cir.
1978) (holding the federal and state tests to be the same), but
we have now firmly come down on the side that the reason-
ableness of a seizure depends exclusively on federal law.3

   In the oft-cited case of United States v. Chavez-Vernaza,
844 F.2d 1368 (9th Cir. 1987), we held that “evidence seized
in compliance with federal law is admissible without regard
to state law.” Id. at 1374; see also United States v. Bynum,
362 F.3d 574, 582-83 (9th Cir. 2004). Of particular signifi-
cance to the resolution of this case is the well-established
proposition that an arrest in violation of state law may still be
constitutionally reasonable. Barry v. Fowler, 902 F.2d 770,
773 (9th Cir. 1990) (no Fourth Amendment violation in
§ 1983 case when plaintiff arrested in violation of a state law).
In Barry, we observed that “[w]hile Barry may have a remedy
under state law, she has failed to allege a federal constitu-
tional . . . violation.” Id.4
  3
     We have not deemed state law wholly irrelevant to Fourth Amendment
analysis. Our precedent supports at least two exceptions to the principle
that compliance with state law does not determine constitutional reason-
ableness, searches incident to arrest and inventory searches. See Cormier,
220 F.3d at 1111-12 (reasonableness of search incident to arrest depends
on legality of arrest under state law and reasonableness of inventory
search depends on compliance with state and local procedures). The stop
of Becerra-Garcia does not fall under either of these exceptions, nor does
the federal test for its validity incorporate state or tribal law. Becerra-
Garcia was the subject of an investigatory traffic stop, the reasonableness
of which depends only on reasonable suspicion, not on compliance with
state or tribal law. See Haynie v. County of Los Angeles, 339 F.3d 1071,
1075 (9th Cir. 2003) (investigatory stops require only reasonable suspi-
cion).
   4
     We acknowledge some inconsistency in our cases on this broad issue.
See, e.g., Bingham v. City of Manhattan Beach, 341 F.3d 939, 950 (9th
1452              UNITED STATES v. BECERRA-GARCIA
   We note that the question of whether and how state law
affects the constitutional reasonableness of a seizure has long
troubled courts and has produced inconsistent results. See
Santoni v. Potter, 369 F.3d 594, 598 (1st Cir. 2004) (stating
that the circuits are split on whether “an arresting officer’s
lack of authority under state or federal law to conduct an oth-
erwise constitutionally valid arrest constitutes an unreason-
able seizure under the Fourth Amendment”); Jackson v.
Louisiana, 980 F.2d 1009, 1011 n.9 (5th Cir. 1993) (citing
conflicting cases to show that whether the unauthorized use of
police powers violates the Fourth Amendment is unresolved).

   The weight of authority establishes that the test of whether
a search or seizure violates the Fourth Amendment “is one of
federal law, neither enlarged by what one state court may
have countenanced, nor diminished by what another may have
colorably suppressed.” Elkins v. United States, 364 U.S. 206,
224 (1960); see also Cooper v. California, 386 U.S. 58, 61
(1967) (whether a search is reasonable under the Fourth
Amendment is a different question than whether a seizure was
authorized by state law). Accordingly, we and several of our
sister circuits have declined to consider state law in determin-
ing the reasonableness of seizures. See United States v. Bell,
54 F.3d 502, 504 (8th Cir. 1995) (“[W]e do not think Fourth
Amendment analysis requires reference to an arrest’s legality
under state law . . . . An arrest by state officers is reasonable
in the Fourth Amendment sense if it is based on probable
cause.”); Fields v. City of South Houston, 922 F.2d 1183,
1189 (5th Cir. 1991) (“[A] civil rights action [under § 1983]

Cir. 2003) (“in evaluating a custodial arrest executed by state officials,
federal courts must determine the reasonableness of the arrest in reference
to state law governing the arrest”) (internal quotation marks omitted);
Pierce v. Multnomah County, 76 F.3d 1032, 1041 (9th Cir. 1996) (holding,
in a § 1983 case, that city policy that authorized officers to detain people
for nonarrestable offenses violated state law and therefore violated the
Fourth Amendment); Reed v. Hoy, 891 F.2d 1421, 1427 n.5 (9th Cir.
1990) (stating that state law is often relevant in analyzing the reasonable-
ness of police conduct).
               UNITED STATES v. BECERRA-GARCIA              1453
will not lie for a warrantless misdemeanor arrest in violation
of state law.”); Chavez-Vernaza, 844 F.2d at 1374 (“evidence
seized in compliance with federal law is admissible without
regard to state law”); Street v. Surdyka, 492 F.2d 368, 372
(4th Cir. 1974) (“Even if Officer Surdyka violated Maryland
arrest law, he cannot be liable under section 1983 unless he
also violated the federal constitutional law governing warrant-
less arrests.”).

  B.   THE SEIZURE WAS REASONABLE

   [6] Having established that, in this case, the legality of the
seizure does not depend on the rangers’ authority under tribal
law, we turn to the question whether, under Fourth Amend-
ment standards, the stop was reasonable. “The Fourth Amend-
ment requires only reasonable suspicion in the context of
investigative traffic stops.” Haynie v. County of Los Angeles,
339 F.3d 1071, 1075 (9th Cir. 2003) (citation and internal
quotations omitted). Because Becerra-Garcia did not chal-
lenge the district court’s finding that the rangers had reason-
able articulable suspicion to investigate for criminal trespass
until his reply brief, we leave that finding undisturbed. See
Indep. Towers of Wash. v. Washington, 350 F.3d 925, 929
(9th Cir. 2003). Notably, the scope of the rangers’ intrusion
was minimal: they merely turned their jeep around, followed
the van, and turned on their emergency lights. Thus, the stop
was reasonable.

   Becerra-Garcia urges us to hold otherwise, suggesting that
we adopt what, at bottom, is a bright-line rule for determining
reasonableness: that a stop is automatically unreasonable if
the officers lacked authority to conduct the seizure. The diffi-
culty with this argument is that it seeks, in effect, to undo the
general principle that federal law, not state or tribal law, gov-
erns the inquiry.

  [7] The reasonableness of a seizure is generally analyzed
on a case-by-case basis, not according to bright-line rules.
1454           UNITED STATES v. BECERRA-GARCIA
See, e.g., Terry v. Ohio, 392 U.S. 1, 17, 20-21 (1968) (reject-
ing a “rigid all-or-nothing model of justification and regula-
tion under the [Fourth] Amendment” in favor of a flexible
model that considers the scope of the intrusion and its justifi-
cation); Go-Bart Importing Co. v. United States, 282 U.S.
344, 357 (1931) (“There is no formula for the determination
of reasonableness. Each case is to be decided on its own facts
and circumstances”). Following the longstanding principle
that reasonableness cannot be reduced to per se rules, we have
never held that a stop that exceeds an officer’s tribal authority
is automatically unreasonable. We decline to embrace
Becerra-Garcia’s sweeping rule because it runs counter to the
flexible notion of reasonableness. The claim that the rangers
lack specific tribal authority to stop vehicles does not trans-
form this otherwise reasonable stop into an unreasonable one.

   Our holding is consistent with our general recognition that
Indian tribes are sovereigns with the power to enforce internal
laws. See, e.g., United States v. Enas, 255 F.3d 662, 666 (9th
Cir. 2001) (en banc). Intrinsic in tribal sovereignty is the
power to exclude trespassers from the reservation, a power
that necessarily entails investigating potential trespassers. See
Ortiz-Barraza v. United States, 512 F.2d 1176, 1179-80 (9th
Cir. 1975); State v. Schmuck, 121 Wash.2d 373, 381 (Wash.
1993) (relying on Ortiz-Barraza to hold that tribal authorities
have the power to stop vehicles to investigate violations of
tribal law). Holding that the minimally intrusive investigatory
stop was reasonable evinces our respect for both the Fourth
Amendment and tribal sovereignty.

  AFFIRMED.
