An ICE detainer, officially called Form I-247A, is a written request from ICE to a jail, prison, or other law enforcement agency. It asks that agency to hold a person for up to 48 additional hours beyond when they would normally be released, so that ICE has time to come and take custody.
There are several critical things to understand about detainers. First, a detainer is a request, not a court order or a warrant. Local law enforcement agencies are not legally required to honor ICE detainers. Whether they comply depends on the policies of that particular jurisdiction. Some cities and counties, often called sanctuary jurisdictions, have policies that limit or prohibit cooperation with ICE detainers. Others comply routinely.
Second, a detainer can be issued at any point during involvement with the criminal justice system, including immediately after arrest when no charges have been filed and before any conviction has occurred. This means a person could be arrested for a minor offense, have the charges dropped, post bail, and still be transferred to ICE custody rather than being released.
Third, a detainer is not the same as a Notice to Appear, which is the document that formally initiates removal proceedings in immigration court. A detainer just holds the person while ICE figures out next steps.
If your family member has an ICE detainer placed on them, contact an immigration attorney as quickly as possible. The 48-hour window is short and early legal intervention matters.