Probable Cause
Rule
A police officer has probable cause to arrest when the facts and circumstances within the officer’s knowledge and of which they had reasonably trustworthy information are sufficient in themselves to warrant a man of reasonable caution in the belief that an offense has been or is being committed (Brinegar). Probable cause to search exists where known facts and circumstances are sufficient to warrant a man of reasonable prudence in the belief that contraband or evidence of a crime will be found in the place to be searched.
Elements / Test
Illinois v. Gates (1983) — Totality of the Circumstances:
- Assess all facts and circumstances known to the officer
- Veracity/reliability of the informant and basis of knowledge are relevant factors but not separate rigid prongs
- Neither factor alone is dispositive; they may compensate for weaknesses in the other
- Standard is a commonsense, nontechnical conception based on practical considerations of everyday life
Prior test — Aguilar-Spinelli Two-Prong Test (still used in many states):
- Reliability/veracity of the informant (track record, corroboration)
- Basis of knowledge (how did informant learn the information?)
Exceptions
- Probable cause is a fluid concept and does not vary with the seriousness of the offense
- Anonymous tips can supply probable cause if sufficiently corroborated (Gates)
- Scope of probable cause is person-specific: probable cause to search a bar does not extend to all patrons (Ybarra)
Policy
Probable cause serves as a substantive floor protecting against arbitrary police intrusions while allowing effective law enforcement. The Gates totality standard replaced the rigid two-prong test to give police and courts more practical flexibility, though critics argue it weakens privacy protections by allowing corroboration of innocent detail to substitute for basis and reliability.
Key Cases
- Brinegar v. United States — defining probable cause as “reasonable caution” standard
- Illinois v. Gates — adopts totality of circumstances; overrules rigid Aguilar-Spinelli test
- Draper v. United States — detailed tip from consistently reliable informant provides probable cause
- Ybarra v. Illinois — probable cause to search premises does not extend to all persons present