Katz was arrested for illegal gambling, he had been gambling over a public phone. The FBI attached an electronic recorder onto the outside of the public phone booth. The state courts claimed this to be legal because the recording device was on the outside of the phone and the FBI never entered the booth. The Supreme Court ruled in the favor of Katz. They stated that the Fourth Amendment allowed for the protection of a person and not just a person's property against illegal searches. .
The Fourth Amendment written in 1791 states, 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 (Had to look that up). The court was unsure on weather or not they should consider a public telephone booth as an area protected by the fourth amendment. The court did state that," The Fourth Amendment protects people, not places". .
What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected. Searches conducted without warrants have been held unlawfully and have not withstood facts, unquestionably showing probable cause, for the Constitution requires that the deliberate impartial judgment of a judicial officer be interposed between the citizen and the police. .
The FBI agents found out the days and times he would use the pay phone. So the FBI attached a tape recorder to the outside of the telephone booth. The FBI recorded him using the phone six different times; all six conversations were around three minutes long.