The petitioner, Charles Katz, was charged with conducting illegal gambling operations across state lines in violation of federal law. In order to collect evidence against Katz, federal agents placed a warrantless wiretap on the public phone booth that he used to conduct these operations. The agents listened only to Katz's conversations, and only to the parts of his conversations dealing with illegal gambling transactions.
In the case of Olmstead v. United States (1928), the Supreme Court held that the warrantless wiretapping of phone lines did not constitute an unreasonable search under the Fourth Amendment. According to the Court, physical intrusion (a trespass) into a given area, and not mere voice amplification (the normal result of a wiretap), is required for an action to constitute a Fourth Amendment search. This is known as the "trespass doctrine." Partly in response to this decision, Congress passed the Federal Communications Act of 1933. This Act required, among other things, federal authorities to obtain a warrant before wiretapping private phone lines. In the case of Silverman v. United States (1961), the Supreme Court refined the Olmstead trespass doctrine by holding that an unreasonable search occurs only if a "constitutionally protected area" has been intruded upon.
At his trial, Katz sought to exclude any evidence connected with these wiretaps, arguing that the warrantless wiretapping of a public phone booth constitutes an unreasonable search of a "constitutionally protected area" in violation of the Fourth Amendment. The federal agents countered by saying that a public phone booth was not a "constitutionally protected area," therefore, they could place a wiretap on it without a warrant.
By a 7-1 vote, the U.S. Supreme Court agreed with Katz and held that placing of a warrantless wiretap on a public phone booth constitutes an unreasonable search in violation of the Fourth Amendment. The majority opinion, written by Justice Potter Stewart, however, did not address the case from the perspective of a "constitutionally protected area." In essence, the majority argued that both sides in the case were wrong to think that the permissibility of a warrantless wiretap depended upon the area being placed under surveillance. "For 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," the Court stated.
Building upon this reasoning, the Court held that it was the duty of the Judiciary to review petitions for warrants in instances in which persons may be engaging in conduct that they wish to keep secret, even if it were done in a public place. The Court held that, in the absence of a judicially authorized search warrant, the wiretaps of the public phone booth used by Katz were illegal. Therefore, the evidence against him gathered from his conversations should be suppressed.
Justice John Marshall Harlan's Concurrence: Test for Constitutionally Protected Searches
Although he agreed with the majority opinion of the Court, Justice Harlan went further to provide a test for what is a constitutionally protected search. He said it was necessary to clarify when private actions, conducted in a public place, may be constitutionally protected. Expanding upon the general principles enunciated by the majority opinion, Justice Harlan proposed the following two-pronged test to address this issue: "My understanding of the rule that has emerged from prior judicial decisions is that there is a twofold requirement, first that a person have exhibited an actual (subjective) expectation of privacy; and second, that the expectation be one that society is prepared to recognize as 'reasonable.'"
Both the Supreme Court and the lower federal courts have looked to this two-pronged test, and not the majority holding per se, to determine when private actions in public places may be constitutionally protected. In essence, this concurrence has come to be seen as the main point of the Katz decision, and it is the test that, typically, has been used when deciding upon the constitutionality of warrantless wiretaps.