Immigration Detainers: Background and Recent Legal Developments [Updated October 9, 2020] [open pdf - 699KB]
From the Document: "The Department of Homeland Security (DHS) has authority to detain non-U.S. nationals (aliens) who are subject to removal. Within DHS, Immigration and Customs Enforcement (ICE) is mainly responsible for the arrest, detention, and removal of aliens found in the interior of the United States. If an alien whom ICE believes to be removable is in custody by state or local law enforcement officers (LEOs), ICE may take custody of the alien through a 'detainer.' A detainer requests that state or local LEOs hold an alien in their custody for up to 48 hours after the alien would otherwise be released to facilitate the alien's removal. ICE's practice of issuing detainers has been subject to legal challenge, including on the ground that the continued detention of an alien pending transfer to ICE custody violates the Fourth Amendment's protections against unreasonable searches and seizures."
CRS Legal Sidebar, LSB10375
Congressional Research Service: https://crsreports.congress.gov/