Late one night Officer James Saylor of the Crow Police  Department approached a truck parked on United States Highway 212, a public  rightof-way within the Crow Reservation in the State of Montana.  Saylor spoke to the driver, Joshua James  Cooley, and observed that Cooley appeared to be non-native and had watery,  bloodshot eyes.  Saylor also noticed two  semiautomatic rifles lying on Cooley's front seat.  Fearing violence, Saylor ordered Cooley out  of the truck and conducted a patdown search.   Saylor also saw in the truck a glass pipe and a plastic bag that  contained methamphetamine.  Additional  officers, including an officer with the federal Bureau of Indian Affairs,  arrived on the scene in response to Saylor's call for assistance. Saylor was  directed  to seize all contraband in  plain view, leading Saylor to discover more methamphetamine. Saylor took Cooley  to the Crow Police Department where federal and local officers further questioned  Cooley. Subsequently, a federal grand jury indicted Cooley on drug and gun  offenses.  The District Court granted  Cooley's motion to suppress the drug evidence. The Ninth Circuit affirmed.  It reasoned that a tribal police officer  could stop (and hold for a reasonable time) a non-Indian suspect if the officer  first tries to determine whether the suspect is non-Indian and, in the course  of doing so, finds an apparent violation of state or federal law. The Ninth  Circuit concluded that Saylor had failed to    make that initial determination here. 
      Held: A tribal  police officer has authority to detain temporarily and to search non-Indian  persons traveling on public rights-of-way running through a reservation for  potential violations of state or federal law.  
      Pp. 3–9.
      (a) As a "general  proposition," the "inherent sovereign powers of an 
      Indian tribe do not extend to the activities of nonmembers of  the tribe."  
      Montana v. United States, 450 U. S. 544, 565.  The Court identified in Montana two exceptions to that general rule, the second of which  fits almost like a glove here: A tribe retains inherent authority over the  conduct of non-Indians on the reservation "when that conduct threatens or has  some direct effect on . . . the health or welfare of the tribe."  Id.,  at 566. The conclusion that Saylor's actions here fall within Montana's second exception is consistent  with the Court's prior Montana cases.  See Strate v. A–1 Contractors, 520 U. S. 438, 456 n. 11; see also Atkinson Trading Co. v. Shirley, 532 U. S. 645, 651.  Similarly, the Court has held that when the  "jurisdiction to try and punish an offender rests outside the tribe, tribal  officers may exercise their power to detain the offender and transport him to  the proper authorities."  Duro v. Reina, 495 U. S. 676, 697.   Ancillary to the authority to transport a non-Indian suspect is the  authority to search that individual prior to transport, as several state courts  and other federal courts have held. While that authority has sometimes been  traced to a tribe's right to exclude non-Indians, tribes "have inherent  sovereignty independent of th[e] authority arising from their power to  exclude," Brendale v. Confederated Tribes and Bands of Yakima  Nation, 492 U. S. 408, 425 (plurality opinion), and here Montana's second exception recognizes  that inherent authority.  In addition,  recognizing a tribal officer's authority to investigate potential violations of  state or federal laws that apply to non-Indians whether outside a reservation  or on a public right-of-way within the reservation protects public safety  without implicating the concerns about applying tribal laws to non-Indians  noted in the Court's prior cases.   Finally, the Court doubts the workability of the Ninth Circuit's  standards, which would require tribal officers first to determine whether a  suspect is non-Indian and, if so, to temporarily detain a non-Indian only for  "apparent" legal violations.  919 F. 3d  1135, 1142.  The first requirement  produces an incentive to lie. The second requirement introduces a new standard  into search and seizure law and creates a problem of interpretation that will  arise frequently given the prevalence of non-Indians in Indian reservations.  
      Pp. 3–7.
      (b) Cooley's  arguments against recognition of inherent tribal sovereignty here are  unpersuasive.  While the Court agrees the  Montana exceptions should not be  interpreted so as to " 'swallow the rule,' " Plains Commerce Bank v. Long  Family Land & Cattle Co., 554 U. S. 316, 330, this case does not raise  that concern due to the close fit between Montana's  second exception and the facts here.  In  addition, the Court sees nothing in existing federal cross-deputization  statutes that suggests Congress has sought to deny tribes the authority at  issue. To the contrary, existing legislation and executive action appear to  operate on the assumption that tribes have retained this authority.  Pp. 8–9. 
      
      919 F. 3d 1135, vacated and remanded. 
      BREYER, J., delivered the opinion for a unanimous  Court.  ALITO, J.,   filed a concurring opinion. 
SUPREME COURT OF THE UNITED STATES
_________________ 
        No. 19–1414 
        _________________ 
        UNITED STATES, PETITIONER v. JOSHUA JAMES COOLEY 
        ON WRIT OF CERTIORARI TO THE UNITED  STATES COURT OF APPEALS FOR THE NINTH CIRCUIT  
        [June 1, 2021] 
        JUSTICE BREYER  delivered the opinion of the Court. 
      The question  presented is whether an Indian tribe's police officer has authority to detain  temporarily and to search a non-Indian on a public right-of-way that runs  through an Indian reservation.  The  search and detention, we assume, took place based on a potential violation of  state or federal law prior to the suspect's transport to the proper nontribal  authorities for prosecution.
        We have previously  noted that a tribe retains inherent sovereign authority to address "conduct  [that] threatens or has some direct effect on . . . the health or welfare of  the tribe." Montana v. United States, 450 U. S. 544, 566  (1981); see also Strate v. A–1 Contractors, 520 U. S. 438, 456, n.  11 (1997). We believe this statement of law governs here.  And we hold the tribal officer possesses the  authority at issue. 
        I 
      Late at night in  February 2016, Officer James Saylor of the Crow Police Department was driving  east on United States Highway 212, a public right-of-way within the Crow  Reservation, located within the State of Montana.  Saylor saw a truck parked on the westbound  side of the highway.  
      Believing the occupants might need assistance, Saylor  approached the truck and spoke to the driver, Joshua James Cooley. Saylor  noticed that Cooley had "watery, bloodshot eyes" and "appeared to be  non-native."  App. to Pet. for Cert. 95a.  Saylor also noticed two semiautomatic rifles lying on the front seat.  Eventually fearing violence, Saylor ordered Cooley out of the truck and  conducted a patdown search.  He called  tribal and county officers for assistance.   While waiting for the officers to arrive, Saylor returned to the truck.  He saw a glass pipe and plastic bag that contained methamphetamine. The other  officers, including an officer with the federal Bureau of Indian Affairs, then  arrived.  They directed Saylor to seize  all contraband in plain view, leading him to discover more  methamphetamine.  Saylor took Cooley to  the Crow Police Department where federal and local officers further questioned  Cooley.
 
      In April 2016, a  federal grand jury indicted Cooley on drug and gun offenses.  See 21 U. S. C. §841(a)(1); 18 U. S. C.  §924(c)(1)(A).  The District Court  granted Cooley's motion to suppress the drug evidence that Saylor had seized.  It reasoned that Saylor, as a Crow Tribe police officer, lacked the authority  to investigate nonapparent violations of state or federal law by a non-Indian  on a public right-of-way crossing the reservation. 
        The Government  appealed.  See 18 U. S. C. §3731.  The Ninth Circuit affirmed the District  Court's evidence- suppression determination. The Ninth Circuit panel wrote that  tribes "cannot exclude non-Indians from a state or federal highway" and "lack  the ancillary power to investigate non-Indians who are using such public rights-of-way."  919 F. 3d 1135, 1141 (2019). It added that a  tribal police officer nonetheless could stop (and hold for a reasonable time) a  non-Indian suspect, but only if (1) the officer first tried to determine  whether "the person is an Indian," and, if the person turns out to be a  non-Indian, (2) it is "apparent" that the person has violated state or federal  law.  Id.,  at 1142.  
        Non-Indian status, the panel added, can usually be  determined by "ask[ing] one question."  Ibid. (internal quotation marks  omitted). Because Saylor had not initially tried to determine whether Cooley  was an Indian, the panel held that the lower court correctly suppressed the  evidence.  The Ninth Circuit denied the  Government's request for rehearing en banc. We then granted the Government's  petition for certiorari in order to decide whether a tribal police officer has  authority to detain temporarily and to search non-Indians traveling on public  rights-of-way running through a reservation for potential violations of state  or federal law. 
II
Long ago we described Indian tribes as "distinct, independent political communities" exercising sovereign authority. Worcester v. Georgia, 6 Pet. 515, 559 (1832). Due to their incorporation into the United States, however, the "sovereignty that the Indian tribes retain is of a unique and limited character." United States v. Wheeler, 435 U. S. 313, 323 (1978). Indian tribes may, for example, determine tribal membership, regulate domestic affairs among tribal members, and exclude others from entering tribal land. See, e.g., Plains Commerce Bank v. Long Family Land & Cattle Co., 554 U. S. 316, 327–328 (2008). On the other hand, owing to their "dependent status," tribes lack any "freedom independently to determine their external relations" and cannot, for instance, "enter into direct commercial or governmental relations with foreign nations." Wheeler, 435 U. S., at 326. Tribes also lack inherent sovereign power to exercise criminal jurisdiction over non- Indians. See Oliphant v. Suquamish Tribe, 435 U. S. 191, 212 (1978). In all cases, tribal authority remains subject to the plenary authority of Congress. See, e.g., Michigan v. Bay Mills Indian Community, 572 U. S. 782, 788 (2014).
      Here, no treaty or  statute has explicitly divested Indian tribes of the policing authority at  issue.  We turn to precedent to determine  whether a tribe has retained inherent sovereign authority to exercise that  power.  In answering this question, our  decision in Montana v. United States, 450 U. S. 544 (1981), is  highly relevant.  In that case we asked  whether a tribe could regulate hunting and fishing by nonIndians on land that  non-Indians owned in fee simple on a reservation. We held that it could not. We  supported our conclusion by referring to our holding in Oliphant that a tribe could not "exercise criminal jurisdiction  over non- Indians." Montana, 450 U.  S., at 565.  We then wrote that the "principles  on which [Oliphant] relied support  the general proposition that the inherent sovereign powers of an Indian tribe  do not extend to the activities of nonmembers of the tribe." Ibid. 
      At the same time, we  made clear that Montana's "general  proposition" was not an absolute rule.  Ibid. We set forth two important  exceptions.  First, we said that a "tribe may regulate, through taxation,  licensing, or other means, the activities of nonmembers who enter consensual  relationships with the tribe or its members, through commercial dealing,  contracts, leases, or other arrangements."   Ibid.  Second,  we said that a "tribe may also retain inherent power to exercise civil  authority over the conduct of nonIndians on fee lands within its reservation when that conduct threatens or has some  direct effect on the political integrity, the economic security, or the health  or welfare of the tribe."  Id., at 566 (emphasis added).
      The second exception  we have just quoted fits the present case, almost like a glove.  The phrase speaks of the protection of the  "health or welfare of the tribe."  To  deny a tribal police officer authority to search and detain for a reasonable time  any person he or she believes may commit or has committed a crime would make it  difficult for tribes to protect themselves against ongoing threats.  Such threats may be posed by, for instance,  non-Indian drunk drivers, transporters of contraband, or other criminal  offenders operating on roads within the boundaries of a tribal  reservation.  As the Washington Supreme  Court has noted, "[a]llowing a known drunk driver to get back in his or her  car, careen off down the road, and possibly kill or injure Indians or non-Indians  would certainly be detrimental to the health or welfare of the Tribe."  State v.  Schmuck, 121 Wash. 2d 373, 391, 850  P. 2d 1332, 1341, cert. denied, 510 U. S. 931 (1993). 
      We have subsequently  repeated Montana's proposition and  exceptions in several cases involving a tribe's jurisdiction over the  activities of non-Indians within the reservation. See, e.g., Plains Commerce Bank, 554 U. S., at 328– 330; Nevada v. Hicks, 533 U. S. 353, 358–360, and n. 3 (2001); South Dakota v. Bourland, 508 U. S. 679, 694–696 (1993); Duro v. Reina, 495 U. S.  676, 687–688 (1990); Brendale v. Confederated Tribes and Bands of Yakima  Nation, 492 U. S. 408, 426–430 (1989) (plurality opinion).  In doing so we have reserved a tribe's  inherent sovereign authority to engage in policing of the kind before us.  Most notably, in Strate v. A–1 Contractors,  520 U. S. 438, 456–459 (1997), we relied upon Montana's general jurisdiction-limiting principle to hold that  tribal courts did not retain inherent authority to adjudicate personal-injury  actions against nonmembers of the tribe based upon automobile accidents that took  place on public rights-of-way running through a reservation. But we also said: 
        "We do not here question the authority of tribal police to  patrol roads within a reservation, including rightsof-way made part of a state  highway, and to detain and turn over to state officers nonmembers stopped on  the highway for conduct violating state law.   Cf. State v. Schmuck, 121 Wash. 2d 373, 390, 850 P.  2d 1332, 1341 (en banc) (recognizing that a limited tribal power 'to stop and  detain alleged offenders in no way confers an unlimited authority to regulate the right of the public to travel  on the Reservation's roads'), cert. denied, 510 U. S. 931 (1993)." 520 U. S.,  at 456, n. 11. 
        We reiterated this point in Atkinson Trading Co. v. Shirley,  532 U. S. 645, 651 (2001), there confirming that Strate "did not question the ability of tribal police to patrol the  highway." 
        Similarly, we  recognized in Duro that "[w]here  jurisdiction to try and punish an offender rests outside the tribe, tribal  officers may exercise their power to detain the offender and transport him to  the proper authorities."  495 U. S., at  697.  The authority to search a  non-Indian prior to transport is ancillary to this authority that we have  already recognized. Cf. Ortiz-Barraza v.  United States, 512 F. 2d 1176,  1180–1181 (CA9 1975).  Indeed, several state courts and other  federal courts have held that tribal officers possess the authority at issue  here. See,  e.g., Schmuck, 121 Wash. 2d, at 390, 850 P. 2d, at 1341; State v. Pamperien, 156 Ore. App. 153, 155–159, 967 P. 2d 503, 504–506  (1998); State v. Ryder, 98 N. M. 453, 456, 649 P. 2d 756, 759 (1982); see also United States v. Terry, 400 F. 3d 575, 579–580 (CA8 2005); Ortiz-Barraza, 512 F. 2d, at 1180–1181; see generally F. Cohen,  Handbook of Federal Indian Law §9.07, p. 773  (2012). To be sure, in Duro we traced  the relevant tribal authority to a tribe's right to exclude non-Indians from  reservation land.  See 495 U. S., at  696–697.  But tribes "have inherent  sovereignty independent of th[e] authority arising from their power to  exclude," Brendale, 492 U. S., at 425  (plurality opinion), and here Montana's  second exception recognizes that inherent authority. 
        We also note that our  prior cases denying tribal jurisdiction over the activities of non-Indians on a  reservation have rested in part upon the fact that full tribal jurisdiction would  require the application of tribal laws to non-Indians who do not belong to the  tribe and consequently had no say in creating the laws that would be applied to  them.  See Duro, 495 U. S., at 693 (noting the concern that tribal-court  criminal jurisdiction over nonmembers would subject such defendants to "trial  by political bodies that do not include them"); Plains Commerce Bank, 554 U. S., at 337 (noting that nonmembers  "have no part in tribal government" and have "no say in the laws and  regulations that govern tribal territory").   Saylor's search and detention, however, do not subsequently subject  Cooley to tribal law, but rather only to state and federal laws that apply  whether an individual is outside a reservation or on a state or federal highway  within it. As the Solicitor General points out, an initial investigation of  non-Indians' "violations of federal and state laws to which those non-Indians  are indisputably subject" protects the public without raising "similar  concerns" of the sort raised in our cases limiting tribal authority.  Brief for United States 24–25. 
        Finally, we have  doubts about the workability of the standards that the Ninth Circuit set  out.  Those standards require tribal  officers first to determine whether a suspect is non-Indian and, if so, allow  temporary detention only if the violation of law is "apparent."  919 F. 3d, at 1142.  The first requirement, even if limited to  asking a single question, would produce an incentive to lie. The second  requirement—that the violation of law be "apparent"—introduces a new standard  into search and seizure law. Whether, or how, that standard would be met is not  obvious.  At the same time, because most  of those who live on Indian reservations are non-Indians, this problem of  interpretation could arise frequently.   See, e.g., Brief for Former  United States Attorneys as Amici Curiae  24 (noting that 3.5 million of the 4.6 million people living in American Indian  areas in the 2010 census were non-Indians); Brief for National Indigenous  Women's Resource Center et al. as Amici  Curiae 19–20 (noting that more than 70% of residents on several  reservations are non-Indian). 
III
In response, Cooley cautions against "inappropriately expand[ing] the second Montana exception." Brief for Respondent 24–25 (citing Atkinson, 532 U. S., at 657, n. 12, and Strate, 520 U. S., at 457–458). We have previously warned that the Montana exceptions are "limited" and "cannot be construed in a manner that would swallow the rule." Plains Commerce Bank, 554 U. S., at 330 (internal quotation marks omitted). But we have also repeatedly acknowledged the existence of the exceptions and preserved the possibility that "certain forms of nonmember behavior" may "sufficiently affect the tribe as to justify tribal oversight." Id., at 335. Given the close fit between the second exception and the circumstances here, we do not believe the warnings can control the outcome.
      Cooley adds that  federal cross-deputization statutes already grant many Indian tribes a degree  of authority to enforce federal law. See Brief for Respondent 28–30; see  generally 25 U. S. C. §§2803(5), (7) (Secretary of the Interior may authorize  tribal officers to "make inquiries of any person" related to the "carrying out  in Indian country" of federal law and to "perform any other law enforcement  related duty"); §2805 (Secretary of the Interior may promulgate rules "relating  to the enforcement of " federal criminal law  in Indian country); 25 CFR §12.21  (2019) (Bureau of Indian Affairs may "issue law enforcement commissions" to  tribal police officers "to obtain active assistance" in enforcing federal  criminal law).  Because Congress has  specified the scope of tribal police activity through these statutes, Cooley  argues, the Court must not interpret tribal sovereignty to fill any remaining  gaps in policing authority. See Brief for Respondent 12.
        We are not convinced  by this argument.  The statutory and  regulatory provisions to which Cooley refers do not easily fit the present  circumstances. They are overinclusive, for instance encompassing the authority  to arrest. See §2803(3). And they are also underinclusive.  Because these provisions do not govern  violations of state law, tribes would still need to strike agreements with a  variety of other authorities to ensure complete coverage.  See Brief for Cayuga Nation et al. as Amici Curiae 7–8, 25–27. More broadly,  cross-deputization agreements are difficult to reach, and they often require  negotiation between other authorities and the tribes over such matters as  training, reciprocal authority to arrest, the "geographical reach of the 
      agreements, the jurisdiction of the parties, liability of  officers performing under the agreements, and sovereign immunity."  Fletcher, Fort, & Singel, Indian Country  Law Enforcement and Cooperative Public Safety Agreements, 89 Mich. Bar J. 42,  44 (2010). 
        In short, we see  nothing in these provisions that shows that Congress sought to deny tribes the  authority at issue, authority that rests upon a tribe's retention of  sovereignty as interpreted by Montana, and  in particular its second exception. To the contrary, in our view, existing  legislation and executive action appear to operate on the assumption that  tribes have retained this authority.   See, e.g., Brief for Current  and Former Members of Congress as Amici  Curiae 23–25; Brief for Former U. S. Attorneys as Amici Curiae 28–29. 
* * *
 For these reasons, we  vacate the Ninth Circuit's judgment and remand the case for further proceedings  consistent with this opinion. 
        
        It is so ordered. 
                                                                             Cite  as: 593 U. S. ____ (2021)                                     
      
      ALITO, J., concurring  
SUPREME COURT OF THE UNITED STATES
_________________ 
      No. 19–1414 
      _________________ 
      UNITED STATES, PETITIONER v. JOSHUA JAMES COOLEY 
      ON WRIT OF CERTIORARI TO THE UNITED  STATES COURT OF APPEALS FOR THE NINTH CIRCUIT  
      [June 1, 2021] 
      JUSTICE ALITO,  concurring. 
    I join the  opinion of the Court on the understanding that it holds no more than the  following: On a public right-of-way that traverses an Indian reservation and is  primarily patrolled by tribal police, a tribal police officer has the authority  to (a) stop a non-Indian motorist if the officer has reasonable suspicion that  the motorist may violate or has violated federal or state law, (b) conduct a  search to the extent necessary to protect himself or others, and (c) if the tribal  officer has probable cause, detain the motorist for the period of time  reasonably necessary for a non-tribal officer to arrive on the scene.  

