Probable cause

Law enforcement
in the United States
Separation of powers
Law enforcement agencies
Types of agency
Types of agent

In United States criminal law, probable cause is the standard by which police authorities have reason to obtain a warrant for the arrest of a suspected criminal or the issuing of a search warrant. It is also the standard by which grand juries issue criminal indictments. The principle behind the standard is to limit the power of authorities to perform random or abusive searches (unlawful search and seizure), and to promote lawful evidence gathering and procedural form during criminal arrest and prosecution. The standard also applies to personal or property searches.[1]

The term comes from the Fourth Amendment of the United States Constitution:

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

Probable in this case may relate to statistical probability or to a general standard of common behavior and customs. The context of the word probable here is not exclusive to community standards, and could partially derive from its use in formal mathematical statistics as some have suggested;[2] but cf. probō, Latin etymology.[3]

In U.S. immigration proceedings, the “reason to believe” standard has been interpreted as equivalent to probable cause.[4]


A common definition is "a reasonable amount of suspicion, supported by circumstances sufficiently strong to justify a prudent and cautious person's belief that certain facts are probably true".[5] Notable in this definition is a lack of requirement for public position or public authority of the individual making the recognition, allowing for use of the term by citizens and/or the general public. The definition of probable cause is, “(A) reasonable ground for supposing that a charge is well-founded” (Merriam-Webster, 2019).

In the context of warrants, the Oxford Companion to American Law defines probable cause as "information sufficient to warrant a prudent person's belief that the wanted individual had committed a crime (for an arrest warrant) or that evidence of a crime or contraband would be found in a search (for a search warrant)". "Probable cause" is a stronger standard of evidence than a reasonable suspicion, but weaker than what is required to secure a criminal conviction. Even hearsay can supply probable cause if it is from a reliable source or supported by other evidence, according to the Aguilar–Spinelli test.

In Brinegar v. United States, the U.S. Supreme Court defines probable cause as "where the facts and circumstances within the officers' knowledge, and of which they have reasonably trustworthy information, are sufficient in themselves to warrant a belief by a man of reasonable caution that a crime is being committed."[6]

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