Can ICE detain undocumented immigrants without a warrant or probable cause?

1. Legal scaffolding: statutory authority vs. criminal warrants

Congress gave immigration officers broad interior arrest powers that differ from criminal warrants: statutes permit ICE officers to "interrogate" and arrest persons believed to be aliens and to execute administrative (ICE) warrants that do not require prior approval by a detached judicial magistrate in the same way criminal warrants do ^{[1] [2]}; the agency itself states it does not need judicial warrants to make arrests and can use administrative processes such as detainers or ICE warrants ^{[5] [6]}.

2. The constitutional check: courts equate “reason to believe” with probable cause

Federal courts have interpreted the statutory "reason to believe" standard for warrantless immigration arrests under section 1357(a) to be effectively equivalent to the Fourth Amendment's probable cause requirement, and the Supreme Court and lower courts have held that more intrusive seizures must satisfy either probable cause (for arrests) or reasonable suspicion (for brief detentions) ^{[1] [2]}; litigation has also required neutral review in some contexts, reflecting judicial oversight of warrantless detentions ^[3].

3. How practice diverges: detainers, interior arrests and “at‑large” operations

In practice ICE pairs statutory authority with administrative tools—detainers, at‑large arrests in public, and worksite actions—and the agency asserts it can arrest without judicial warrants in many interior settings, while acknowledging limits for private homes or non‑public areas where judicial warrants or consent are required ^{[5] [7] [8]}. Detainers are supposed to be based on probable cause determinations, according to ICE guidance, but critics and courts have found detainer-based holds can lack the procedural protections found in criminal arrests ^{[6] [3]}.

4. Recent courtroom constraints and consent decrees

Litigation has pushed back: a federal consent decree in Illinois and subsequent court orders have limited ICE's ability to make warrantless arrests in certain operations and required transparency and relief for people allegedly detained without probable cause, and appeals courts have at times kept those restraints in place while disputes proceed ^{[4] [9]}. Separately, federal appeals rulings have demanded neutral, prompt review when detentions are premised on ICE detainers, highlighting constitutional deficiencies in how detainers were used ^[3].

5. Operational realities and policy implications

ICE detains large numbers of people through both custodial pipelines (criminal referrals) and resource‑intensive at‑large arrests; data show many detained people have no criminal conviction, underscoring that administrative immigration detention is broad in scope even as courts scrutinize probable cause and procedural safeguards ^{[10] [11]}. Agency statements and litigation posture reflect competing agendas: ICE emphasizes statutory authority and public‑safety rationales ^{[5] [6]}, while immigrant‑rights groups and civil‑liberties litigants press constitutional limits and challenge detainers and warrantless arrests as prone to error and abuse ^{[3] [4]}.

6. Bottom line and limits of available reporting

The bottom line is that ICE can and does make many warrantless arrests under immigration law, but those arrests are legally constrained by statutory standards interpreted through the Fourth Amendment's probable cause and by ongoing court orders and consent decrees that have curtailed certain warrantless practices in specific jurisdictions; reporting and case law show both agency assertions of broad authority and judicial pushback requiring probable cause and neutral review in some contexts ^{[1] [2] [4] [3]}. This analysis is limited to the provided sources and does not account for any additional, more recent statutes, cases, or localized policies outside those documents.