Fourth Amendment of the United States Constitution

The Fourth Amendment (Amendment IV) to the United States Constitution is part of the Bill of Rights that prohibits unreasonable searches and seizures. It requires "reasonable" governmental searches and seizures to be conducted only upon issuance of a warrant, judicially sanctioned by probable cause, supported by oath or affirmation, particularly describing the place to be searched and the persons or things to be seized. Under the Fourth Amendment, search and seizure (including arrest) should be limited in scope according to specific information supplied to the issuing court, usually by a law enforcement officer who has sworn by it. Fourth Amendment case law deals with three issues: what government activities constitute "search" and "seizure"; what constitutes probable cause for these actions; and how violations of Fourth Amendment rights should be addressed. Early court decisions limited the amendment's scope to a law enforcement officer's physical intrusion onto private property, but with Katz v. United States (1967), the Supreme Court held that its protections, such as the warrant requirement, extend to the privacy of individuals as well as physical locations. Law enforcement officers need a warrant for most search and seizure activities, but the Court has defined a series of exceptions for consent searches, motor vehicle searches, evidence in plain view, exigent circumstances, border searches, and other situations.

The exclusionary rule is one way the amendment is enforced. Established in Weeks v. United States (1914), this rule holds that evidence obtained through a Fourth Amendment violation is generally inadmissible at criminal trials. Evidence discovered as a later result of an illegal search may also be inadmissible as "fruit of the poisonous tree", unless it inevitably would have been discovered by legal means.

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