An ICE detainer (Form I‑247) is a written request from ICE asking a state or local jail or law‑enforcement agency to (1) notify ICE before releasing a person and (2) hold the person for up to 48 hours (excluding weekends/holidays) so ICE can assume custody. It is a request, not a judicial warrant, and by itself does not give ICE additional arrest authority or automatically convert someone������'s local detention into federal custody—the detainer derives from immigration statutes/regulations (8 C.F.R. §287.7) and ICE������'s authority to detain removable noncitizens, but it does not itself supersede state/local detention law.