"An 'immigration detainer' is a document by which U.S. Immigration and Customs Enforcement (ICE) advises other law enforcement agencies of its interest in individual aliens whom these agencies are detaining. 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, the nationwide implementation of the Secure Communities program between 2008 and 2013 has prompted numerous questions about detainers. This program relies upon information sharing between various levels and agencies of government to identify potentially removable aliens. Detainers may then be issued for these aliens. Prior to 1986, the Immigration and Nationality Act (INA) did not explicitly address detainers, and the INS appears to have issued detainers pursuant to its 'general authority' to guard U.S. borders and boundaries against the illegal entry of aliens, among other things. However, in 1986, Congress amended the INA to address the issuance of detainers for aliens arrested for controlled substance offenses. After the 1986 amendments, INS promulgated two regulations, one addressing the issuance of detainers for controlled substance offenses and the other addressing detainers for other offenses. These regulations were merged in 1997 and currently address various topics, including who may issue detainers and the temporary detention of aliens by other law enforcement agencies. There is also a standard detainer form (Form I-247) that allows ICE to indicate that it has taken actions that could lead to the alien's removal, and request that another agency take actions that could facilitate such removal (e.g., notify ICE before the alien's release)."
CRS Report for Congress, R42690