Authority of State and Local Police to Enforce Federal Immigration Law [September 10, 2012] [open pdf - 354KB]
From the 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. […] 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."
CRS Report for Congress, R41423