Search Warrants and Their Exceptions

The Fourth Amendment protects against unreasonable searches and seizures and requires that warrants be issued only on probable cause, supported by oath, and describing the place to be searched and the things to be seized. Federal agents apply to a judge, often a magistrate judge, with a sworn affidavit. Evidence obtained illegally is generally excluded from trial, a rule applied to federal courts in 1914 and to the states in Mapp v. Ohio in 1961, and since Katz v. United States in 1967 the amendment has protected reasonable expectations of privacy, not just property.

Courts allow searches without a warrant in many situations: when a person consents, when evidence is in plain view, in emergencies such as hot pursuit or danger to life, during an arrest, for vehicles when there is probable cause, for brief stops and pat-downs on reasonable suspicion under Terry v. Ohio in 1968, and at the border. The home receives the strongest protection, and officers generally need a warrant to enter one to make an arrest. Immigration officers usually act on administrative warrants signed by immigration officials rather than judges, and whether those can justify entering a home without consent was disputed in 2025 and 2026.

Technology keeps testing the rules. The Supreme Court held in Riley v. California in 2014 that police need a warrant to search a cell phone seized during an arrest and in Carpenter v. United States in 2018 that they need one to obtain historical cell phone location records, though it has kept the doctrine that information shared with companies such as banks gets less protection. Courts are divided over geofence warrants and forensic searches of phones at the border, and agencies have bought location data from commercial brokers without warrants. The House passed a bill to bar such purchases in April 2024, but the Senate did not act. Law enforcement groups say flexibility is needed to solve crimes, while civil liberties groups say warrants should be the rule for digital data.