probable cause

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Probable cause is a requirement found in the Fourth Amendment that must usually be met before police make an arrest, conduct a search, or receive a warrant. Courts usually find probable cause when there is a reasonable basis for believing that a crime may have been committed (for an arrest) or when evidence of the crime is present in the place to be searched (for a search). Under exigent circumstances, probable cause can also justify a warrantless search or seizure. Persons arrested without a warrant are required to be brought before a competent authority shortly after the arrest for a prompt judicial determination of probable cause.


Constitutional Basis

Although the Fourth Amendment states that "no warrants shall issue, but upon probable cause", it does not specify what "probable cause" actually means. The Supreme Court has attempted to clarify the meaning of the term on several occasions, while recognizing that probable cause is a concept that is imprecise, fluid and very dependent on context. In Illinois v. Gates, the Court favored a flexible approach, viewing probable cause as a "practical, non-technical" standard that calls upon the "factual and practical considerations of everyday life on which reasonable and prudent men [...] act".1 Courts often adopt a broader, more flexible view of probable cause when the alleged offenses are serious.

Application to Arrests

The Fourth Amendment requires that any arrest be based on probable cause, even when the arrest is made pursuant to an arrest warrant. Whether or not there is probable cause typically depends on the totality of the circumstances, meaning everything that the arresting officers know or reasonably believe at the time the arrest is made.2 However, probable cause remains a flexible concept, and what constitutes the “totality of the circumstances” often depends on how the court interprets the reasonableness standard.3

A lack of probable cause will render a warrantless arrest invalid, and any evidence resulting from that arrest (physical evidence, confessions, etc.) will have to be suppressed.4 A narrow exception applies when an arresting officer, as a result of a mistake by court employees, mistakenly and in good faith believes that a warrant has been issued. In this case, notwithstanding the lack of probable cause, the exclusionary rule does not apply and the evidence obtained may be admissible.5 Unlike court clerks, prosecutors are part of a law enforcement team and are not "court employees" for purposes of the good-faith exception to the exclusionary rule.6

Application to Search Warrants

Probable cause exists when there is a fair probability that a search will result in evidence of a crime being discovered.7 For a warrantless search, probable cause can be established by in-court testimony after the search. In the case of a warrant search, however, an affidavit or recorded testimony must support the warrant by indicating on what basis probable cause exists.8

A judge may issue a search warrant if the affidavit in support of the warrant offers sufficient credible information to establish probable cause.9 There is a presumption that police officers are reliable sources of information, and affidavits in support of a warrant will often include their observations.10 When this is the case, the officers’ experience and training become relevant factors in assessing the existence of probable cause.11 Information from victims or witnesses, if included in an affidavit, may be important factors as well.12

The good faith exception that applies to arrests also applies to search warrants: when a defect renders a warrant constitutionally invalid, the evidence does not have to be suppressed if the officers acted in good faith.13 Courts evaluate an officer’s good faith by looking at the nature of the error and how the warrant was executed.14

Probable Cause in the Digital Age

While the Fourth Amendment's probable cause requirement has historically been applied to physical seizures of tangible property, the issue of searches and seizures as applied to data has come to the Supreme Court's attention in recent years.

In Riley v California (2014), the Supreme Court held: "The police generally may not, without a warrant, search digital information on a cellphone seized from an individual who has been arrested." This would seem to group cell phones in with traditional items subject to traditional court tests and rules for searches and seizures.

Riley, however, did not end the inquiry into digital data's interaction with the Fourth Amendment. For the 2018 term, the Supreme Court has agreed to hear Carpenter v. United States. Carpenter, accused of several robberies, was arrested after "his phone company shared data on his whereabouts with law-enforcement agents."

Mr. Carpenter is challenging the "constitutionality of the Stored Communications Act, a law permitting phone companies to divulge information when there are 'specific and articulable facts' that are 'relevant and material' to a criminal investigation." His complaint states that "his privacy rights under the Fourth Amendment were violated when his phone company shared data on his whereabouts with law-enforcement agents." This case will likely have a significant impact on the role that probable cause plays in the ability of data companies to share user information with law enforcement.

  • 1. See Illinois v. Gates, 462 U.S. 213, 232 (1983).
  • 2. United States v. Humphries, 372 F.3d 653, 657 (4th Cir. 2004).
  • 3. Prosecutor's Manual for Arrest, Search and Seizure, § 6-6(b) (2004).
  • 4. See Mapp v. Ohio, 367 U.S. 643 (1961), at 648, 655.
  • 5. See Ariz. v. Evans, 514 U.S. 1 (1995).
  • 6. People v. Boyer, 305 Ill. App. 3d 374 (1999), at 379-80.
  • 7. See Gates, 462 U.S. at 238.
  • 8. Whiteley v. Warden, 401 U.S. 560, 564 (1971).
  • 9. Prosecutor's Manual for Arrest, Search and Seizure, § 3-2(c) (2004).
  • 10. See Franks v. Delaware, 438 U.S. 154, 171 (1978).
  • 11. See United States v. Mick, 263 F.3d 553, 566 (6th Cir. 2001).
  • 12. See United States v. Schaefer, 87 F.3d 562, 566 (1st Cir. 1996).
  • 13. See United States v. White, 356 F.3d 865 (8th Cir. 2004).
  • 14. See, e.g., United States v. Clark, 638 F.3d 89, 100–05 (2d Cir. 2011)