	Introduction

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. 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. Moreover, alien removal (deportation) and associated administrative processes are civil in nature. 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. States can therefore be precluded from taking actions that are otherwise within their authority if federal law would thereby be thwarted. 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]. Major Judicial Rulings Concerning Immigration Enforcement by State and Local Police

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. A few states subsequently passed measures that authorized state police to arrest certain categories of aliens who committed immigration status violations making them removable. 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. 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. 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."
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].