"An 'immigration detainer' is a document that advises other law enforcement agencies that the Department of Homeland Security (DHS)--and specifically U.S. Immigration and Customs Enforcement (ICE)--seeks custody of an alien whom they are holding in order to arrest and remove the alien. ICE and its predecessor, the Immigration and Naturalization Service (INS), have used detainers as one means of obtaining custody of aliens for removal proceedings since at least 1950. However, implementation of the Secure Communities program has recently prompted numerous questions about detainers. This program relies upon information sharing between various levels and agencies of government to identify potentially removable aliens. […] Some commentators and advocates for immigrants' rights have asserted that, because the INA [Immigration and Nationality Act] addresses only detainers for controlled substance offenses, ICE's detainer regulations and practices are beyond its statutory authority and, thus, unlawful. The only court to address this issue found otherwise, but several recently filed cases raise the issue anew in other jurisdictions."
CRS Report for Congress, R42690