An ICE immigration detainer (DHS Form I‑247/I‑247A or successors) is a federal request that asks a jail or law enforcement agency to (1) notify ICE before releasing a person believed removable and (2) hold that person up to 48 hours so ICE can assume custody. It is a request, not a judicial warrant; federal regulations and ICE guidance treat detainers as non‑mandatory, and multiple federal courts have held that a detainer alone does not provide independent legal authority to continue holding someone without separate probable cause or state law authority.