An ICE “arrest detainer” (Form I-247 or similar) is a written request from ICE asking a jail, prison or law-enforcement agency to (1) notify ICE before releasing a person believed to be removable and (2) hold that person up to 48 hours beyond release so ICE can assume custody. It is an administrative request, not a judicial arrest warrant, and does not itself create new legal authority to arrest or detain under federal law; detainers ‘take effect’ only when served and if a jail holds someone beyond the 48‑hour window they may be held liable. (ICE policy also requires a probable‑cause finding before issuing a detainer.)
Governors and mayors don’t have to “accept” ICE detainers; state and local officials decide whether their jails/prisons will honor ICE’s requests. ICE detainers are non‑binding requests under federal law and courts (e.g., 3d Cir. in Galarza v. Szalczyk) have confirmed local agencies are not required to comply and can be liable for unlawful detention if they hold someone without a constitutionally valid basis. A governor or mayor can direct local policy on cooperation (subject to state law and any local agreements like 287(g)).
DHS’s phrase “criminal illegal aliens” is a policy term for noncitizens whom DHS/ICE has identified as removable and who have been arrested, charged or convicted of criminal offenses. ICE typically prioritizes lodging detainers when there is a criminal conviction or when the person poses a public‑safety or national‑security threat; issuance requires an ICE finding—usually probable cause—that the person is removable. Determinations rely on local criminal records, conviction data, database checks, and ICE review.
The DHS press release does not publish a line‑by‑line methodology. When agencies report counts like “1,360 detainers” or “nearly 470 released,” those figures ordinarily come from ICE’s enforcement/ERO records/databases (internal detainer and custody logs, IAQ/IDENT checks and ERO statistics). DHS/ICE did not provide the underlying dataset or exact query in the release, so the agency’s precise calculation and the time frame/filters aren’t publicly documented in that statement.
Practical consequences vary: (a) operational — ICE may need to locate and arrest the person in the community instead of taking custody at release (ICE says at‑large arrests are harder and potentially more dangerous); (b) legal — jurisdictions that do honor detainers can face civil liability if they detain people without a constitutionally valid basis (see Galarza); (c) political/financial — disputes over cooperation can affect federal–local relations and grant discussions, but courts have held detainers are requests, not mandates. There is no automatic criminal penalty for refusing a detainer; legal exposure is mostly about either unlawful detention (if you hold beyond the allowed time) or constitutional claims.
Roles differ: ICE (federal) identifies removable noncitizens, issues detention requests (detainers), and—through ERO—takes custody, detains and pursues removal. State and local law enforcement arrest and hold people for violations of state/local law; they can choose to notify/hold for ICE but their authority to detain comes from state law and the Fourth Amendment. Only ICE officers (and designated 287(g) officers) may issue detainers; honoring a detainer is a local decision and not itself a federal warrant.