probable cause
Probable cause is a requirement under the Fourth Amendment that must generally be satisfied before police may make an arrest, conduct a search, or obtain a warrant. Probable cause exists when the facts and circumstances within an officer’s knowledge would lead a reasonable person to believe that a crime has been committed (for an arrest) or that evidence of a crime is present in a specific location (for a search).
Probable cause requires more than reasonable suspicion, which permits only brief, limited investigative stops and pat-downs when an officer can point to specific, articulable facts suggesting that criminal activity may be occurring. Probable cause, by contrast, demands a higher degree of certainty and a fair probability that evidence or criminal conduct will be found. Under exigent circumstances, probable cause can justify a warrantless search or seizure. A person arrested without a warrant must be promptly brought before a judge or magistrate for a determination of probable cause.
Constitutional Basis
The Fourth Amendment provides that “no warrants shall issue, but upon probable cause,” yet does not define the term. The Supreme Court has described probable cause as a flexible, context-dependent concept. In Illinois v. Gates, 462 U.S. 213 (1983), the Court characterized it as a “practical, non-technical” standard based on “factual and practical considerations of everyday life on which reasonable and prudent men act.” Courts often apply a broader interpretation when the alleged offenses are serious.
Application to Arrests
An arrest must be supported by probable cause, whether or not it is made pursuant to a warrant. The determination depends on the totality of the circumstances known to the officer at the time of the arrest. See United States v. Humphries, 372 F.3d 653 (4th Cir. 2004). A warrantless arrest lacking probable cause is invalid, and evidence derived from it must be suppressed. See Mapp v. Ohio, 367 U.S. 643 (1961). A narrow exception to this exclusionary rule applies when an officer acts in good faith reliance on an erroneous court record indicating that a warrant exists. In such a case, evidence obtained need not be excluded. See Arizona v. Evans, 514 U.S. 1 (1995). This good-faith exception does not extend to prosecutors, who are considered part of law enforcement rather than court personnel. See People v. Boyer, 305 Ill. App. 3d 374 (1999).
Application to Search Warrants
Probable cause exists when there is a fair probability that a search will uncover evidence of a crime. See Gates, 462 U.S. at 238. For warrantless searches, probable cause may be demonstrated through later testimony; for warrant-based searches, it must be supported by an affidavit or recorded testimony describing the basis for the belief. See Whiteley v. Warden, 401 U.S. 560 (1971). A judge may issue a search warrant when the supporting affidavit contains credible facts establishing probable cause. Affidavits frequently rely on police observations, and officers’ training and experience are relevant in assessing credibility. See Franks v. Delaware, 438 U.S. 154 (1978); United States v. Mick, 263 F.3d 553 (6th Cir. 2001). Information from victims or witnesses may also be used. See United States v. Schaefer, 87 F.3d 562 (1st Cir. 1996).
The good-faith exception that applies to arrests also applies to defective warrants – evidence obtained under an invalid warrant may be admissible if officers acted in objectively reasonable reliance on it. See United States v. White, 356 F.3d 865 (8th Cir. 2004); United States v. Clark, 638 F.3d 89 (2d Cir. 2011).
Probable Cause in the Digital Age
While the Fourth Amendment’s probable cause standard was developed in the context of physical searches and seizures, courts have extended it to digital evidence. In Riley v. California, 573 U.S. 373 (2014), the Supreme Court held that police generally may not, without a warrant, search digital information on a cell phone seized during an arrest. The Court recognized the heightened privacy interests associated with modern electronic devices. In Carpenter v. United States, 585 U.S. 296 (2018), the Court held that accessing historical cell-site location information constitutes a search under the Fourth Amendment, requiring a warrant supported by probable cause. The decision limited the government’s ability to obtain digital data based only on “specific and articulable facts” under the Stored Communications Act. Carpenter significantly expanded Fourth Amendment protection to digital records held by third parties.
[Last reviewed in October of 2025 by the Wex Definitions Team]
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