The power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, and primarily with Congress. Concomitant to its exclusive power to establish rules which determine which aliens may enter and which may stay in the country, the federal government also has the power to proscribe activities that subvert this system and establish penalties for those who undertake prohibited activities. These powers have primarily been implemented through the Immigration and Nationality Act of 1952, as amended (INA). The INA establishes a comprehensive set of requirements for legal immigration, naturalization, and the removal of aliens, as well as rules governing aliens' continued presence in the United States. The INA also establishes an enforcement regime to deter violations of federal immigration law, including through the imposition of penalties upon persons who violate INA requirements. In examining the INA, it is crucial to distinguish between its civil and criminal provisions. For example, the INA generally makes it a criminal offense for an alien to enter the United States without authorization, with heightened penalties available in cases where an alien unlawfully reenters after having previously been ordered removed from the country. Moreover, persons who transport unauthorized aliens into or within the United States, or harbor such aliens within the country, are generally subject to criminal penalty. On the other hand, some violations of the INA are subject to civil penalties. For example, an entity that knowingly hires an alien who is not authorized to work in the United States may be subject to a civil monetary penalty. Moreover, alien removal (deportation) and associated administrative processes are civil in nature. For example, an alien's unauthorized immigration status makes him removable, but absent additional factors (e.g., having reentered the United States after being formally removed), unlawful presence does not constitute a criminal offense. In some cases, conduct may potentially be subject to both civil and criminal sanction under the INA. For instance, an alien who unlawfully enters the United States may be subject to criminal penalty as well as deportation. However, the fact that an alien may be subject to both criminal sanction and removal for an immigration violation does not mean that each tool shall be employed. Congressional authority to prescribe rules on immigration does not necessarily imply exclusive authority to enforce those rules. Congress may expressly authorize states and localities to assist in enforcing federal law. Moreover, there is a notion that has been articulated in some federal courts and by the executive branch that states may possess "inherent" authority to assist in the enforcement of federal immigration law, even in the absence of express authorization by federal statute. Nonetheless, state enforcement of federal immigration law must always be consistent with federal authority. The Supremacy Clause of the Constitution establishes that federal law, treaties, and the Constitution itself are "the supreme Law of the Land." States can therefore be precluded from taking actions that are otherwise within their authority if federal law would thereby be thwarted. Congressional intent is paramount in the analysis as to whether federal law preempts state or local activity; accordingly, a court must determine whether Congress expressly or implicitly intended to preempt state or local action. Generally, a court will determine that Congress intended to preempt state regulation or activity when (1) Congress expresses preemptive intent in "explicit statutory language"; (2) a state entity regulates "in a field that Congress intended the Federal Government to occupy exclusively"; or (3) a state entity's activity "actually conflicts with federal law." A question of ongoing legal dispute concerns the extent to which state and local law enforcement may be preempted from directly enforcing federal immigration law in the absence of express authorization by federal statute. Recently, several states have enacted measures to facilitate the detection of unlawfully present aliens by state and local law enforcement officials. Many of these measures are the subject of ongoing litigation. The U.S. Department of Justice (DOJ), in particular, has challenged measures enacted by several states which are intended to deter the presence of unlawfully present aliens within their jurisdiction. In a 2012 ruling in the case of Arizona v. United States , the Supreme Court ruled that one such measure enacted by Arizona, commonly referred to as S.B. 1070, was largely preempted by federal immigration law. In the course of its decision, the Court indicated that states' ability to enforce federal immigration law, at least as it pertains to non-criminal immigration status violations, is limited in the absence of either direct authorization by federal law or coordination of enforcement efforts with federal authorities. This report discusses the authority of state and local law enforcement to assist in the enforcement of federal immigration law through the investigation and arrest of persons believed to have violated such laws. It describes current provisions in federal law that permit state and local police to enforce immigration law directly; analyzes major cases concerning the ability of states and localities to assist in immigration enforcement, including the Supreme Court's ruling in Arizona v. United States ; and briefly examines opinions on the issue by the Office of Legal Counsel (OLC) within the Department of Justice. This report does not discuss legal issues raised by state and local measures intended to supplement federal immigration laws through the imposition of additional criminal or civil penalties. For more discussion of the legal implications of such measures, see CRS Report R42719, Arizona v. United States: A Limited Role for States in Immigration Enforcement , by [author name scrubbed] and [author name scrubbed], and CRS Report R41991, State and Local Restrictions on Employing Unauthorized Aliens , by [author name scrubbed]. The enforcement of federal immigration law by state and local police is most clearly permissible when Congress has evidenced intent to authorize such activity. In exercising its power to regulate immigration, Congress is free to delegate to the states, among other things, the authority to arrest, hold, and transport aliens into federal custody. Indeed, Congress has created several avenues for states and localities to assist in the enforcement of federal immigration law. The following sections discuss notable provisions in federal statutes that expressly authorize state and local law enforcement to directly engage in immigration enforcement activities, including arresting persons who have violated federal immigration law. This section does not discuss those provisions of federal law that, while contemplating participation by state and local authorities in immigration enforcement matters (such as the sharing of immigration status information between federal, state, and local authorities) , do not directly authorize state and local police to perform immigration enforcement duties. One of the broadest grants of authority for state and local immigration enforcement activity stems from Section 133 of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA), which amended INA Section 287 to permit the delegation of certain immigration enforcement functions to state and local officers. Pursuant to INA Section 287(g), the Attorney General (now the Secretary of Homeland Security ) is authorized to enter into a written agreement with a State, or any political subdivision of a State, pursuant to which an officer or employee of the State or subdivision, who is determined by the [Secretary of Homeland Security] to be qualified to perform a function of an immigration officer in relation to the investigation, apprehension, or detention of aliens in the United States (including the transportation of such aliens across State lines to detention centers), may carry out such function at the expense of the State or political subdivision and to the extent consistent with State and local law. Agreements entered pursuant to INA Section 287(g) (commonly referred to as "287(g) agreements") enable specially trained state or local officers to perform specific functions relative to the investigation, apprehension, or detention of aliens, during a predetermined time frame and under federal supervision. In order for state or local officers to perform functions pursuant to a 287(g) agreement, they must "have knowledge of and adhere to" federal law governing immigration officers and be certified as having received "adequate training" regarding the enforcement of immigration laws. State or local officers performing functions pursuant to 287(g) agreements are not considered federal employees, except for purposes relating to certain tort claims and compensation matters, but are considered to be acting under color of federal law for purposes of liability and immunity from suit in any civil actions brought under federal or state law. INA Section 287(g)(10) specifies that a written agreement is not required for state or local officials to engage in certain cooperative functions with federal immigration authorities (though these officials would not be entitled to the same rights and immunities as persons operating under a 287(g) agreement). Specifically, no agreement is necessary for a state or local officer to communicate with federal authorities concerning the immigration status of any person, including persons believed to be unlawfully present in the United States. More broadly, no agreement is necessary in order for a state or local officer "otherwise to cooperate … in the identification, apprehension, detention, or removal of aliens not lawfully present in the United States." An unsettled issue concerning state efforts to enforce federal immigration law is whether the "cooperation" contemplated under INA Section 287(g)(10) requires states and localities to consult and coordinate their immigration enforcement efforts with federal authorities. However, in Arizona v. United States , discussed infra , the Supreme Court stated that although "[t]here may be some ambiguity as to what constitutes cooperation under the federal law[,] … no coherent understanding of the term would incorporate the unilateral decision of state officers to arrest an alien for being removable absent any request, approval, or other instruction from the Federal Government." The 287(g) agreements follow two different models. Under the jail enforcement model (also referred to as the "detention model"), designated officers within state or local detention facilities are authorized to identify and process criminal aliens in preparation for removal by federal immigration authorities. Under the task force model, designated officers may, during the course of their regular law enforcement duties within the community or under the direction of a supervising federal immigration officer, identify and arrest certain removable aliens. Some 287(g) agreements singularly employ a task force or detention model, while others use both. In 2009, U.S. Immigration and Customs Enforcement (ICE), the agency within the Department of Homeland Security which administers the 287(g) program, renegotiated agreements with participating jurisdictions in an effort to bolster federal oversight, training, and communication within the 287(g) program, and to prioritize the arrest and detention of aliens involved in serious criminal activity. As of August 31, 2012, agreements pursuant to INA Section 287(g) were in place with 64 law enforcement agencies within 24 states. It should be noted that federal immigration authorities have entered cooperative arrangements with states pursuant to statutory authorities other than INA Section 287(g). For example, under the Criminal Alien Program (CAP), ICE officers assigned to federal, state, and local prisons are tasked with identifying criminal aliens in order to facilitate their removal, including through the placement of detainers upon such aliens so that federal immigration authorities may take them into custody upon completion of their criminal sentences. A separate program, Secure Communities, is also used to identify criminal aliens in local law enforcement custody. This program—which was first implemented in 14 jurisdictions in 2008 and is scheduled for implementation nationwide in 2013—relies upon the sharing of information regarding persons arrested by state and local law enforcement to identify aliens who may be removable. Specifically, the fingerprints of persons arrested by state and local officers are sent to the Federal Bureau of Investigation's (FBI's) Integrated Automatic Fingerprint Identification System (IAFIS), which then sends them to ICE's Automated Biometric Identification System (IDENT). This system automatically notifies ICE personnel whenever the fingerprints of persons arrested by state and local officers match those of a person previously encountered and fingerprinted by immigration officials. ICE personnel then review other databases to determine whether the person is here illegally or otherwise removable, and may issue detainers for any aliens who appear removable. Unlike 287(g) agreements, neither CAP nor the Secure Communities initiative involves direct enforcement of federal immigration law by state or local law enforcement officers or agencies. Moreover, whereas CAP is effectuated via formal arrangements between federal and state authorities, the Secure Communities program, though initially effectuated through written agreements between ICE and state identification bureaus, is now implemented through an information-sharing arrangement between federal authorities. In addition to formal agreements, federal immigration authorities sometimes have informal cooperative arrangements with state or local law enforcement, particularly along the northern and southern borders, in which officers will provide support to one another in the performance of their law enforcement duties. Section 372 of IIRIRA amended INA Section 103(a) to authorize the Attorney General (now the Secretary of Homeland Security ) to call upon state and local police to perform immigration enforcement functions in response to an actual or imminent mass influx of aliens. Specifically, INA Section 103(a) provides: In the event that the [Secretary of Homeland Security] determines that an actual or imminent mass influx of aliens arriving off the coast of the United States or near a land border presents urgent circumstances requiring an immediate Federal response, the [Secretary] may authorize any State or local law enforcement officer, with the consent of the head of the department, agency or establishment under whose jurisdiction the individual is serving, to perform or exercise any of the power, privileges or duties conferred or imposed by the Act or regulations issued thereunder upon officers or employees of the service. Thus, state and local officers may exercise the civil or criminal arrest powers of federal immigration officers when certain criteria are met: (1) the designated state and local officers are expressly authorized by the Secretary of Homeland Security to exercise such authority; (2) the head of the relevant state or local law enforcement agency has given its consent to the performance of federal immigration functions by the agency's officers; and (3) the Secretary has made a determination that an imminent or ongoing mass influx of aliens requires an immediate response. Any authority delegated to state or local law enforcement officers under this provision can only be exercised for the duration of the emergency. In 2002, the DOJ issued a final rule that implemented INA Section 103(a)(10) and described the cooperative process by which state or local governments could agree to place authorized state and local law enforcement officers under the direction of the INS in exercising federal immigration enforcement authority. The following year the DOJ found it necessary to amend the previous regulations, determining that the regulations did not provide the Attorney General with sufficient flexibility to address unanticipated situations that might occur during a mass influx of aliens. When such action is deemed necessary to protect public safety, public health, or national security, the new rules also allow the abbreviation or waiver of training requirements for state and local law enforcement. Although one preemptory agreement was entered with Florida pursuant to INA Section 103(a)(1) in 1998, which could go into effect in the event that a mass influx of aliens is declared, it does not appear that any other agreements have been entered pursuant to this authority. Section 439 of the Antiterrorism and Effective Death Penalty Act of 1996 (AEDPA, P.L. 104-132 ) authorizes state and local law enforcement officers to arrest unlawfully present criminal aliens who have presumably violated INA Section 276 (concerning the reentry of previously removed aliens). Section 439 states in part: [T]o the extent permitted by relevant State and local law, State and local law enforcement officials are authorized to arrest and detain an individual who—(1) is an alien illegally present in the United States; and (2) has previously been convicted of a felony in the United States and deported or left the United States after such conviction, but only after the State or local law enforcement officials obtain appropriate confirmation from the Immigration and Naturalization Service of the status of such individual and only for such period of time as may be required for the Service to take the individual into Federal custody for purposes of deporting or removing the alien from the United States. This provision originated as a floor amendment during congressional consideration of AEDPA, and its sponsor intended it to overcome a perceived federal limitation on state and local officers' ability to arrest and detain criminal aliens so that they could be transferred to the custody of federal immigration authorities. There is some debate as to whether such a limitation actually existed prior to the enactment of AEDPA, and whether states and localities are now only permitted to arrest and detain aliens on account of their unlawful reentry pursuant to the procedure established under AEDPA Section 439 (i.e., when state or local officers have obtained prior confirmation of a suspect's unauthorized immigration status from federal immigration authorities). As discussed infra , the U.S. Court of Appeals for the Ninth Circuit appears to have construed AEDPA Section 439 in this manner, while the U.S. Court of Appeals for the Tenth Circuit has recognized that federal law pre-AEDPA was not intended to displace any preexisting authority permitting states and localities to enforce federal immigration law. The Supreme Court did not squarely assess the intended effect of AEDPA Section 439 in its 2012 ruling in Arizona v. United States , though it cited the provision as one of the few avenues through which state and local police could make arrests on the basis of aliens' suspected removability. Congress appears to have authorized state and local police to enforce INA Section 274, which criminalizes activities relating to the smuggling, transport, or harboring of unauthorized aliens. INA Section 274(c), entitled "Authority to Arrest," states that "No officer or person shall have authority to make any arrest for a violation of any provision of this section except officers and employees of the Service designated by the Attorney General, either individually or as a member of a class, and all other officers whose duty it is to enforce criminal laws ." The plain language in this subsection seems to indicate that state and local law enforcement officers are permitted to make arrests for violations of the federal alien smuggling statute, as they are "officers whose duty it is to enforce criminal laws." The legislative history of INA Section 274 seems to confirm this understanding. The Senate-passed version of this provision stated that arrests for violations could only be made by federal immigration agents and "other officers of the United States whose duty it is to enforce criminal laws." The House, however, struck the words "of the United States," so that state and local officials could enforce this provision as well. Although the federal alien smuggling provision appears to permit state and local officials to directly enforce its provisions, other INA provisions which criminalize immigration-related conduct do not contain similar authorizing language. Nonetheless, as discussed infra , reviewing courts have thus far recognized that state and local law enforcement may arrest persons for criminal violations of the INA, regardless of whether the applicable INA provision expressly authorizes such arrests. The Supreme Court in Arizona declined to definitively resolve this issue. At least until the Supreme Court's decision in Arizona v. United States , there had been considerable debate concerning the power of state and local police to enforce federal immigration law in the absence of express authorization in federal statute. For decades, the prevailing view had been that states were not precluded from arresting persons for criminal violations of the INA, but that they were generally preempted from arresting persons for civil violations making them removable. More recently, however, some courts appeared to take the view that state and local police could generally arrest persons for either criminal violations of federal immigration laws or civil violations making them removable. A few states subsequently passed measures that authorized state police to arrest certain categories of aliens who committed immigration status violations making them removable. Many of these measures were subsequently challenged in federal court on preemption grounds. The Supreme Court agreed to review one such challenge, concerning a comprehensive immigration enforcement measure enacted by Arizona, and held that states are generally preempted from arresting or detaining aliens on the basis of suspected removability under federal immigration law. It should be noted that inquiries by state and local law enforcement that touch upon the immigration status of stopped individuals do not always constitute attempts to enforce federal immigration law. Such inquiries might arise in the normal course of an investigation unrelated to immigration enforcement. For example, an officer investigating an offense under state or local law might question a person regarding his identity, and such questioning might possibly touch upon that person's immigration status (e.g., requesting the production of any documents that may verify the person's purported identify, including perhaps any federal immigration documents in the person's possession). These situations might not raise the same legal issues as situations where questioning regarding immigration status either serves as the legal justification for a person's initial stop, detention, or arrest, or constitutes a basis for detaining a person beyond the period necessary to resolve any non-immigration related matters that justified the person's stop or detention. In June 2012, the Supreme Court issued its decision in Arizona v. United States , ruling that some aspects of an Arizona law intended to deter unlawfully present aliens from remaining in the state were preempted by federal law, but also holding that Arizona police were not facially preempted from running immigration status checks on persons stopped for state or local offenses (though the Court left the door open for future challenges to this provision). The Court's ruling indicates that states' ability to enforce federal immigration law is limited in the absence of either direct authorization by federal law or coordination of enforcement efforts with federal immigration authorities. The Arizona measure, commonly referred to as S.B. 1070, was enacted in 2010 and almost immediately challenged by the DOJ on the grounds that it conflicted with federal immigration law and policy and was therefore unenforceable under the Supremacy Clause. A federal district court preliminarily enjoined enforcement of four of the five provisions of S.B. 1070 that were challenged by the DOJ, pending a final ruling in the case, and the injunction was upheld by a three-judge panel of the U.S. Court of Appeals for the Ninth Circuit. Arizona appealed this ruling to the Supreme Court. Arguments at the Supreme Court centered on four major provisions of S.B. 1070, which can be divided into two categories: (1) those provisions seeking to bolster direct enforcement of federal immigration law by Arizona law enforcement, including through the identification and apprehension of unlawfully present aliens; and (2) those provisions that criminalize conduct which may facilitate the presence of unauthorized aliens within the state. The Arizona Court found that the provisions imposing state criminal penalties on two types of immigration-related conduct were preempted by federal law. However, the Court split on the permissibility of the two provisions of S.B. 1070 involving direct enforcement of federal immigration law by Arizona police. The Court held that Arizona police were generally preempted from arresting or detaining aliens solely on the basis of immigration status violations. However, state and local police were not facially preempted from running immigration status checks with federal authorities when they reasonably suspected a person stopped for a state or local offense was a removable alien. A five-Justice majority ruled that Section 6 of S.B. 1070, which authorized the warrantless arrest of aliens who committed certain criminal offenses that constitute grounds for removal under federal law, was facially preempted. Writing for the majority, Justice Kennedy found that Section 6 would confer broader authority to Arizona police to arrest aliens on the basis of removability than is granted to federal immigration authorities under federal law. The majority also deemed it significant that the arrest authority conferred on Arizona police could be "exercised without any input" from federal authorities, which would "allow the State to achieve its own immigration policy" and potentially lead to unnecessary harassment of certain aliens who were unlikely to be removed by federal authorities. More broadly, the majority recognized that states and localities "may perform the functions of an immigration officer" only in "limited circumstances" specified by federal law. In particular, the Supreme Court held that states are generally preempted from arresting and detaining persons for suspected immigration status violations, except when done pursuant to (1) a written agreement under INA Section 287(g); (2) some other specific federal statutory authorization; or (3) pursuant to a "request, approval, or instruction from the Federal Government." The scope of activities permitted under the third category seems likely to be the subject of continued debate. Three Justices dissented from this portion of the Court's ruling, and would have recognized that state and local police are generally not precluded from assisting in the enforcement of federal immigration law, including by arresting aliens on the basis of their removability under federal law. It is important to note that the Arizona Court's discussion of states' limited authority to enforce federal immigration law was in reference to arrests for immigration status violations, which are non-criminal in nature. The Court did not opine as to whether state law enforcement agencies are also precluded from making arrests for criminal violations of federal immigration law. As previously mentioned, reviewing courts have generally recognized that state and local police are not preempted from making such arrests. Still, the Arizona Court appeared to leave the door open to a possible preemption challenge in the event that a person is arrested or detained by state authorities based on "reasonable suspicion of illegal entry or another immigration crime." Although a majority of the Arizona Court found that most of the challenged provisions of S.B. 1070 were preempted by federal law, the sitting Justices unanimously agreed that Section 2(b) of S.B. 1070, which requires Arizona police, whenever practicable, to investigate the immigration status of persons reasonably suspected of being unlawfully present when such persons are stopped for a state or local offense, is not facially preempted. The Court emphasized that federal law contemplates the sharing of immigration information among federal, state, and local authorities. Similar reasoning was employed by the Court last year, when it rejected a facial preemption challenge to another Arizona law that required the use of a federal work authorization database and imposed licensing sanctions upon Arizona businesses that hire unauthorized aliens, after the Court found that federal law either permitted or encouraged the type of regulation that had been adopted by the state. Nonetheless, a majority of the Arizona Court appeared to take the view that while state police may ask the federal government about the immigration status of stopped individuals, future challenges to the provision might be made depending upon how the requirement was interpreted and applied (e.g., if Arizona police delayed the release of persons in their custody "for no reason other than to verify their immigration status"). Following the Arizona ruling, lower courts have applied the Supreme Court's decision to state laws modeled after Arizona's S.B. 1070. In August 2012, for example, the U.S. Court of Appeals for the Eleventh Circuit ("Eleventh Circuit") issued decisions concerning immigration enforcement measures adopted by Alabama and Georgia soon after the enactment of the Arizona statute. While finding that some provisions of these measures were likely preempted by federal immigration law, the Eleventh Circuit ruled that a provision in the Georgia statute which authorized state police to run immigration status checks on persons stopped for state offenses (which had been modeled on Arizona's S.B. 1070) was not facially preempted, though the court left the door open for a future challenge in the event that the checks result in the prolonged detention of persons. Prior to the Supreme Court's ruling in Arizona , there were conflicting judicial opinions regarding the degree to which state and local police officers could, in the absence of express authorization by federal law, act to enforce federal immigration law. The U.S. Court of Appeals for the Ninth Circuit ("Ninth Circuit") and the U.S. Court of Appeals for the Sixth Circuit ("Sixth Circuit") issued opinions which recognized that state and local law enforcement agencies were generally preempted from making arrests for civil violations of the INA in the absence of clear authorization under federal law, though each either expressly or impliedly endorsed the notion that states were not barred from arresting persons for criminal violations of federal immigration law. On the other hand, the U.S. Court of Appeals for the Tenth Circuit ("Tenth Circuit") issued a series of rulings which appeared to support the position that state and local law enforcement have implicit authority to investigate and arrest persons for either criminal or civil violations of federal immigration law. The continuing validity of these rulings, and in particular those of the Tenth Circuit, is uncertain in the aftermath of the Supreme Court's ruling in Arizona . Nonetheless, it is possible that these decisions may still provide some guidance to future courts and policymakers, particularly to the extent that they may be viewed as supplementing the Court's analysis of immigration enforcement activity by the states, or addressing matters not squarely addressed by the Arizona ruling, including the authority of state and local police to make arrests for criminal violations of federal immigration law. The issue of whether state and local law enforcement agencies are precluded from enforcing provisions of the INA was analyzed by the Ninth Circuit in the 1983 case of Gonzales v. City of Peoria and the 2011 case of United States v. Arizona (which was reviewed by the Supreme Court on appeal as Arizona v. United States , discussed supra ). In Gonzales , a three-judge panel examined a Peoria policy that authorized local officers to arrest aliens who violated INA Section 275, which makes it a criminal offense for an alien to enter the United States unlawfully. The petitioners, who had been questioned and detained pursuant to the city's policy, claimed that enforcement of federal immigration laws was the exclusive responsibility of the federal government, precluding any concurrent enforcement activities by states or localities. The appellate court disagreed. As an initial matter, the Gonzales court noted that the "general rule is that local police are not precluded from enforcing federal statutes," and that federal regulation of a particular field "should not be presumed to preempt state enforcement activity 'in the absence of persuasive reasons—either that the nature of the regulated subject matter permits no other conclusion, or that the Congress has unmistakably so ordained.'" The court concluded that the enforcement of the criminal provisions of the INA by states and localities did not inherently conflict with federal interests. Moreover, the court found that neither the structure nor legislative history of the INA manifested an intent by Congress to preclude state or local enforcement of the INA's criminal provisions. Accordingly, the Gonzales court declared that local police officers may, subject to state law, constitutionally stop or detain individuals when there is reasonable suspicion or, in the case of arrest, probable cause that such persons have violated, or are in the process of violating, the criminal provisions of the INA. In the course of its analysis of the preemptive effect of federal immigration law, the Gonzales court appeared to distinguish the preemptive effect of the INA's civil and criminal provisions, and assumed that the former constituted a pervasive and preemptive regulatory scheme, whereas the latter did not. The court stated: We assume that the civil provisions of the [INA], regulating authorized entry, length of stay, residence status, and deportation, constitute such a pervasive regulatory scheme, as would be consistent with the exclusive federal power over immigration. However, this case does not concern that broad scheme, but only a narrow and distinct element of it—the regulation of criminal immigration activity by aliens. The statutes relating to that element are few in number and relatively simple in their terms. They are not, and could not be, supported by a complex administrative structure. It therefore cannot be inferred that the federal government has occupied the field of criminal immigration enforcement. Whereas the Ninth Circuit had "assumed" in Gonzales that state and local police were precluded from directly enforcing the civil provisions of federal immigration law, a more definitive pronouncement to that effect was made by the circuit court in Arizona . As discussed earlier, the case centered on an Arizona law intended to deter the entry or presence of aliens within the state who lack lawful status under federal immigration law. The DOJ and a number of private entities filed separate lawsuits to prevent aspects of the Arizona statute from going into effect. The reviewing federal district court granted the federal government's request to preliminarily enjoin most of the challenged provisions of the Arizona law, after it found that the DOJ was likely to prevail in its argument that these provisions were preempted by federal immigration law and policy. On appeal, a three-judge panel of the Ninth Circuit affirmed the lower court's ruling. While unanimous on some aspects of its decision, the panel split on the permissibility of those provisions of S.B. 1070 concerning immigration status verifications and warrantless arrests of deportable aliens by state and local police. By a 2-1 decision, the panel held that state and local police officers generally lacked the authority to enforce the non-criminal provisions of the INA, and also held that states may not mandate that police investigate the immigration status of persons suspected of being unlawfully present aliens. In reaching this conclusion, the panel majority construed those provisions of INA Section 287(g) permitting state and local officers to perform immigration enforcement functions pursuant to a written agreement with the Secretary of Homeland Security as indicative of congressional intent for state involvement in immigration enforcement to generally occur under federal supervision. The majority further found that INA Section 287(g)(10), which refers to state and local "cooperation" in immigration enforcement in the absence of a 287(g) agreement, encompasses only assistance on "an incidental and as needed basis" when requested by the Secretary of Homeland Security or otherwise necessary. The majority also viewed the Arizona statute as being inconsistent with congressional intent, on the ground that it would permit state and local police to arrest aliens for civil deportation violations in a broader set of circumstances than had been authorized under AEDPA Section 439. The panel majority further found that the government was likely to prevail in its preemption challenge because of the Arizona measure's "deleterious effect" on foreign relations, as well as "the threat of 50 states layering their own immigration enforcement rules on top of the INA." In a partial dissent from the majority's ruling, one member of the panel criticized the majority for holding that states and localities are generally preempted from enforcing the civil provisions of the INA or investigating the immigration status of persons suspected of being deportable aliens. The dissent characterized most jurisprudence as supporting the proposition that state and local officers are generally not preempted from making arrests for violations of federal law, including arrests for immigration violations. The dissent also construed INA Section 287(g)(10) as reflecting congressional recognition that state police may assist in the enforcement of both the civil and criminal provisions of the INA, even in the absence of a 287(g) agreement. The dissent further claimed that AEDPA Section 439 was not intended to define the parameters of state authority to arrest aliens for civil immigration violations. The dissenting judge also construed 8 U.S.C. Section 1373(c), which requires federal authorities to respond to immigration status requests by state and local authorities, as being indicative of congressional support for state and local participation in immigration enforcement activities. It should be noted that while the circuit panel majority found that state and local police are generally preempted from making arrests for civil violations of the INA, its related ruling that S.B. 1070's immigration verification requirements were preempted (a ruling subsequently overruled by the Supreme Court) appeared to be based on the "mandatory" nature of these requirements. Thus, the court's decision would not necessarily have barred Arizona law enforcement from attempting to verify the immigration status of persons on a more limited, case-by-case basis. The panel majority's opinion apparently contemplated such attempts at verification in limited circumstances. The DOJ also seemed to suggest in its argument before the district court that it did not view discretionary attempts by state or local law enforcement to verify the immigration status of individuals as raising the same preemption concerns as Arizona's "mandatory" requirements relating to status verification. As discussed supra , the Ninth Circuit's ruling was subsequently reviewed by the Supreme Court. Although the Court agreed with the Ninth Circuit regarding the preemptive effect that federal law had upon many provisions of S.B. 1070, it disagreed that the mandatory immigration status requirements imposed by the Arizona law were facially preempted. In the 2008 case of United States v. Urrieta , a three-judge circuit panel similarly appeared to construe federal immigration law as generally precluding states and localities from arresting or detaining persons for civil immigration violations. The case concerned the lawfulness of the petitioner's extended detention following the issuance of a traffic citation by local law enforcement, during which time the officer attempted to determine whether the petitioner was an unlawfully present alien. During the extended detention, the petitioner consented to a search of his vehicle, which resulted in the discovery of firearms and fraudulent documents. In his subsequent criminal trial for unlawful possession of these items, the petitioner sought to have the evidence discovered during his extended detention suppressed, arguing that his extended detention beyond the period necessary to issue a traffic citation was unlawful. The circuit panel concluded that the petitioner's extended detention could not be justified solely on account of the police officer's reasonable suspicion that the petitioner was an unlawfully present alien. In so doing, the panel characterized INA Section 287(g) as "stating that local law enforcement officers cannot enforce completed violations of civil immigration law (i.e., illegal presence) unless specifically authorized to do so by the Attorney General under special conditions that are not applicable in the present case." Although the majority opinion in Urrieta appeared to recognize that state or local law enforcement could detain a person on account of a criminal violation of the INA, it indicated that an alien could not be detained solely on account of unauthorized immigration status in the absence of a 287(g) agreement or other express federal authority. Because the local officer did not have "reasonable suspicion that [the petitioner] was engaged in some nonimmigration-related illegal activity" that could justify his extended detention, the court ruled that the petitioner was unlawfully detained and ordered the evidence discovered during this detention to be suppressed in subsequent criminal proceedings. In contrast to the approach taken by the Sixth and Ninth Circuits, the Tenth Circuit Court of Appeals issued a series of rulings that arguably supported the view that state and local officers were not preempted from investigating and arresting persons who have violated either the criminal or civil provisions of the INA. Although these cases arose in the context of criminal investigations, they concerned activities undertaken by state or local officers involving the enforcement of the civil provisions of federal immigration law—namely, the arrest or extended detention of persons in order to determine whether they were unlawfully present aliens. In the 1984 case of United States v. Salinas-Calderon , a three-judge circuit panel considered a case involving a state trooper who had pulled over the criminal defendant for driving erratically, and who had subsequently found six individuals in the back of the defendant's truck. Because neither the driver nor the six individuals spoke English or carried identification documentation, and another passenger (the driver's wife) stated that they were from Mexico, the state trooper arrested them and attempted to verify their immigration status. The driver was subsequently charged with the criminal offense of unlawfully transporting unauthorized aliens, but moved to suppress statements made by himself and the six passengers in which they admitted their unauthorized immigration status. Examining the record, the circuit panel found that, based on the observable facts that had been available, the trooper had probable cause to detain and arrest all of the individuals. Moreover, the court rejected the defendant's argument that the state trooper lacked authority to detain the passengers in order to inquire into their immigration status. The court determined that a "state trooper has general investigatory authority to inquire into possible immigration violations," and that based on his questioning of the defendant and passengers, the trooper had "probable cause to make a warrantless arrest for violation of the immigration laws." In 1999, the Tenth Circuit Court of Appeals once again considered state and local authority to enforce federal immigration laws in the case of United States v. Vasquez-Alvarez . The case concerned an Oklahoma police officer's arrest of an individual, who was being monitored by the officer partially due to suspicion of drug trafficking, following the individual's admission that he was an "illegal alien." Subsequently, the alien admitted that he had a felony record and had previously been deported from the United States, and was charged by federal authorities with the criminal offense of unlawfully reentering the United States. As discussed previously, Section 439 of AEDPA expressly permits state and local law enforcement to arrest previously deported aliens who have been convicted of criminal activity and thereafter unlawfully reenter the United States, but requires that law enforcement acting pursuant to this authority first obtain confirmation of the alien's immigration status prior to making an arrest. In the instant case, however, the law enforcement officer did not act pursuant to the authority conferred under AEDPA Section 439. Instead, the arrest was premised upon Oklahoma state law, which permitted state and local law enforcement to make arrests for any violation of federal law. The Vasquez-Alvarez court rejected the defendant's argument that because his arrest was not in accordance with the procedure detailed in AEDPA Section 439, it was therefore unlawful. Citing Salinas-Calderon , the circuit court noted that it had previously "held that state law-enforcement officers have the general authority to investigate and make arrests for violations of federal immigration laws." Examining the language and legislative history of AEDPA Section 439, the court determined that the provision neither expressly nor implicitly limited or displaced "the preexisting general authority of state or local police officers to investigate and make arrests for violations of federal law, including immigration law." Instead, the circuit panel held that AEDPA Section 439 "merely creates an additional vehicle for the enforcement of federal immigration law," besides any independent authority to make such arrests under state law. In the 2001 case of United States v. Santana-Garcia , the Tenth Circuit once again addressed the role of state and local law enforcement in immigration matters, reaffirming and expanding upon its prior rulings in Salinas-Calderon and Vasquez-Alvarez . The case concerned a traffic stop by a Utah state trooper. The driver of the car did not possess a driver's license, a misdemeanor under Utah law, and did not speak English. The passenger in the car spoke limited English and explained that he and the driver were traveling from Mexico to Colorado, which prompted the officer to ask if they were "legal." The passenger and the driver appeared to understand the question and answered "no." Following further inquiry, the driver and passenger consented to a search of their vehicle, which revealed illegal drugs. In subsequent criminal proceedings, the driver and passenger moved to suppress this evidence on the grounds that the police lacked reasonable suspicion to detain them beyond the purpose of the initial stop. The circuit panel upheld the admission of the evidence, finding that the state trooper had probable cause to arrest the defendants for violations of state criminal law (i.e., driving without a valid driver's license) and federal law at the time they consented to a search of the vehicle. With respect to federal law, the court held that the defendants' admission of unlawful status provided the state officer with probable cause to arrest them for suspected violations of federal immigration law. The Santana-Garcia panel also seemed to dismiss the suggestion that state law must explicitly authorize state and local officials to make such arrests. The court relied upon a number of inferences from earlier decisions that recognized the "implicit authority" or "general investigatory authority" of state officers to inquire into possible immigration violations. The court also seemed to rely upon a broad understanding of a Utah state law that empowers officers to make warrantless arrests for any public offense committed in the officer's presence to include violations of federal law. Although the defendants in Santana-Garcia were apparently in violation of a civil provision of the INA (i.e., unauthorized presence), the Santana-Garcia court made no distinction between state and local police officers' ability to enforce either the civil or criminal provisions of federal immigration law, although the supporting cases which the court cited generally involved arrests for criminal matters. Moreover, it remains unclear how the court, pursuant to its broad understanding of Utah state law, would have ruled if there had not been an independent legal basis supporting the state officer's stop (i.e., a traffic violation) unrelated to the investigation as to whether a civil violation of federal immigration laws had occurred. In any event, the Supreme Court's decision in Arizona would seem to preclude arguments that state and local police may independently act to enforce federal immigration law through the arrest and detention of persons for non-criminal immigration status violations. In recent decades, the executive branch has repeatedly opined on the scope of potential state and local involvement in the enforcement of federal immigration law. Over the years, it has modified its views as to whether state and local officials may enforce the civil provisions of the INA. In a 1978 press release, the DOJ "reaffirmed … that the enforcement of the immigration laws rests with [federal immigration authorities], and not with state and local police." The DOJ further urged state and local police not to "stop and question, detain, arrest, or place an 'immigration hold' on any persons not suspected of crime, solely on the ground that they may be deportable aliens." In 1983, the DOJ announced revisions to this policy to encourage greater involvement by state and local police in the enforcement of immigration laws, but emphasized that federal authorities "remain responsible for all arrests for [civil] immigration violations." In 1989, the DOJ's OLC opined that while state and local law enforcement could enforce the provisions of the INA concerning criminal offenses, it was "unclear" whether they could enforce non-criminal federal statutes. In 1996, the OLC reached a more definitive conclusion on the question, issuing an opinion which found that while state and local police are not preempted from making arrests for criminal violations of the INA, they "lack recognized legal authority" to enforce the INA's civil provisions. The opinion acknowledged that "[i]t is well-settled that state law enforcement officers are permitted to enforce federal statutes where such enforcement activities do not impair federal regulatory interests." Such enforcement is "subject to the provisions and limitations of state law." However, the OLC concluded, based upon an examination of jurisprudence, that "state and local police lack recognized legal authority to stop and detain an alien solely on suspicion of civil deportability, as opposed to a criminal violation of the immigration laws or other laws." In particular, the OLC construed the Ninth Circuit's ruling in Gonzales v. City of Peoria as holding that state and local authority to enforce the INA "is limited to criminal violations." In 2002, the OLC issued a memorandum which concluded that "federal law did not preempt state police from arresting aliens on the basis of civil deportability," and it withdrew the advice of the 1996 opinion which had suggested otherwise. The 2002 OLC Opinion described the states, like the federal government, as possessing the status of "sovereign entities." Because of this status, states do not require affirmative delegation of federal authority in order to make arrests for violations of federal law—"[i]nstead, the power to make arrests inheres in the ability of one sovereign to accommodate the interests of the other." The 2002 OLC Opinion recognized that the exercise of states' inherent authority to arrest persons for federal violations may be subject to federal preemption. However, it concluded that "federal law should be presumed not to preempt this arrest authority," because "it is ordinarily unreasonable to assume that Congress intended to deprive the federal government of whatever assistance States may provide in identifying and detaining those who have violated federal law." The 2002 OLC Opinion explicitly rejected the 1996 opinion's conclusion that federal law preempts state or local enforcement of the civil provisions of the INA, because "[o]n re-examination, we believe that the authorities we cited in the 1996 OLC opinion provide no support for our conclusion that state police lack the authority to arrest aliens solely on the basis of civil deportability." In particular, it construed the Ninth Circuit's statements in Gonzales v. City of Peoria regarding the preemptive nature of the INA's civil provisions as "mere assumption in dictum ," and instead emphasized Tenth Circuit jurisprudence supporting the inherent authority of state and local police to enforce both the criminal and civil provisions of federal immigration law. Some critics of the 2002 OLC Opinion have characterized it as "deeply flawed" and unsupported by judicial precedent or historical practice in the field of immigration. For example, even prior to the Supreme Court's ruling in Arizona , some critics argued that immigration has long been understood to be a distinctly federal concern, and that Congress would not have provided express statutory authorization for state and local enforcement of civil immigration laws in limited circumstances (e.g., pursuant to INA Section 287(g)) unless it was understood that state and local police were otherwise preempted from making arrests for civil immigration violations. It should be noted that the 2002 OLC Opinion concerned whether states are preempted from arresting persons for violations of federal immigration law. The opinion characterized this as "an extremely limited … preemption question," which does not, "[u]nlike the typical preemption scenario," involve a state enacting its own immigration-related measures, which might "arguably conflict with federal law or intrude into a field that is reserved to Congress or that federal law has occupied." OLC opinions are generally viewed as providing binding interpretive guidance for executive agencies and reflecting the legal position of the executive branch, but they cannot compel state action and do not have the same weight as an act of Congress. Generally, courts will consider opinion letters by executive agencies on legal matters to the extent that they "have the power to persuade." It remains to be seen whether the OLC will modify or supplement any of the conclusions reached in its 2002 opinion, in light of the Supreme Court's ruling in Arizona. The Court's opinion indicates that state and local police do not enjoy broad discretion to determine when and whether to arrest or detain persons for immigration status violations. If such enforcement activity is not directly authorized by federal statute, it must still, at minimum, be pursuant to the "request, approval, or instruction from the Federal Government."

Summary: The power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, and primarily with Congress. Concomitant to its exclusive power to determine which aliens may enter and which may stay in the country, the federal government also has the power to proscribe activities that subvert this system. Congress has defined our nation's immigration laws in the Immigration and Nationality Act (INA), a comprehensive set of laws governing legal immigration, naturalization, work authorization, and the entry and removal of aliens. These requirements are bolstered by an enforcement regime containing both civil and criminal provisions. Deportation and associated administrative processes related to the removal of aliens are civil in nature, while certain violations of federal immigration law, such as smuggling unauthorized aliens into the country, carry criminal penalties. Congressional authority to prescribe rules on immigration does not necessarily imply exclusive authority to enforce those rules. In certain circumstances, Congress has expressly authorized states and localities to assist in enforcing federal immigration law. Moreover, there is a notion that has been articulated in some federal courts and by the executive branch that states may possess "inherent" authority to assist in the enforcement of federal immigration law, even in the absence of clear authorization by federal statute. Nonetheless, states may be precluded from taking actions if federal law would thereby be thwarted. At least until the Supreme Court's decision in the 2012 case of Arizona v. United States, there had been considerable legal debate concerning the power of state and local police to enforce federal immigration law in the absence of express authorization in federal statute. For decades, the prevailing view had been that states were not precluded from arresting persons for criminal violations of the INA, but were generally preempted from arresting persons for civil violations making them removable. More recently, however, some courts (and the Department of Justice (DOJ) in a 2002 legal opinion) took the view that state and local police were not preempted from arresting persons for any violation of federal immigration law, including immigration status violations. A few states subsequently passed measures that authorized state police to arrest certain categories of aliens who committed immigration status violations making them removable. In Arizona, however, the Supreme Court held that states are generally preempted from arresting or detaining aliens on the basis of suspected removability under federal immigration law. Such action may be taken only when there is specific federal statutory authorization, or pursuant to "request, approval, or instruction from the Federal Government." This report discusses the authority of state and local law enforcement to assist in the enforcement of federal immigration law through the investigation and arrest of persons believed to have violated such laws. It describes federal statutes that expressly permit state and local police to enforce immigration law directly, and discusses the Supreme Court's ruling in Arizona v. United States and significant, pre-Arizona lower court decisions concerning the ability of states and localities to assist in immigration enforcement. The report also briefly examines pre-Arizona opinions on the issue by the DOJ's Office of Legal Counsel. This report does not directly address legal issues raised by states and localities enacting their own immigration-related sanctions, including measures intended to supplement federal law through the imposition of additional criminal or civil penalties. For further discussion of the legal implications of such measures, see CRS Report R42719, Arizona v. United States: A Limited Role for States in Immigration Enforcement, by [author name scrubbed] and [author name scrubbed], and CRS Report R41991, State and Local Restrictions on Employing Unauthorized Aliens, by [author name scrubbed].