A "search" occurs for Fourth Amendment purposes when the government intrudes upon an individual's reasonable expectation of privacy. The Fourth Amendment protects individuals from unreasonable searches and seizures by the government, and it requires that law enforcement officers have probable cause and a warrant to search an individual or their property, unless an exception to the warrant requirement applies.

There are several factors that can be considered in determining whether a search has occurred for Fourth Amendment purposes. These include:

  1. Whether the individual had a subjective expectation of privacy in the place or thing searched

  2. Whether society is willing to recognize that expectation of privacy as reasonable

  3. Whether the government's intrusion on the individual's expectation of privacy was substantial

If a search is found to have occurred, the government must show that it had probable cause to conduct the search and that it obtained a warrant, or that an exception to the warrant requirement applies. If the search was unreasonable, any evidence obtained as a result of the search may be excluded from evidence in court.

This case the addresses the following issues: Katz v. United States, 389 U.S. 347 (1967).

  1. Does the Fourth Amendment only prohibit information gathered by trespass?

  2. Can a search occur without a physical intrusion?

This case dealt with two related issues stemming from what exactly the Fourth Amendment prohibits. Id. at 349. Prior to this case, the Fourth Amendment had been held to prevent information gathered by trespassing of government officials. Id. at 351. Instead, the Court shifted the focus to whether a “reasonable expectation of privacy” existed, ultimately delineating a two-prong inquiry: the individual must actual hold an expectation of privacy and it must an expectation society is prepared to recognized as reasonable. Id. at 351-52. Once this privacy has been established, a search occurs when this privacy is invaded, whether physically or otherwise. Id. at 353.

The petitioner here was convicted of “transmitting wagering information by telephone” in federal district court. Id. at 348. His conviction was largely supported by recordings that had been made by FBI agents using a recording device attached to the outside of a public phone booth. Id. The conviction was upheld on the basis that a search requires a physical intrusion, and the attachment of the device to the outside of a phone booth did not have such an intrusion and therefore was no search at all. Id. at 349-50. An additional issue was placed before the Court regarding whether an individual could even expect privacy in a public phone booth. Id. at 350.

The Court dispatched the incorrect notion that the Fourth Amendment is measured in terms of “constitutionally protected areas.” Id. at 351. Instead, “the Fourth Amendment protects people, not places.” Id. Determining when the government is forbidden from intruding without a warrant or a narrow exception to that requirement requires looking at both the subjective expectations of the individual and what society thinks of such a belief. Id. at 350-51. First, the individual him- or herself must have an expectation that actions, items, or activities are private, by “seeking to preserve [them] as private.” Id. at 352. Second, the expectation must be reasonable, in that society agrees with the expectation, meaning the individual “may rely upon the protection of the Fourth Amendment.” Id. In this case, the Court found that entering a phone booth and closing the door is sufficient indication of both a personal expectation of privacy and a reasonable expectation in the eyes of the public at large. Id. Having found this reasonable expectation of privacy, the next part of this inquiry was whether a “search” had occurred despite the lack of physical intrusion. Id.

The previous decisions on what constituted a search had set a threshold question of whether a “physical penetration” occurred. Id. In both Olmstead v. United States, 277 U.S. 438 (1928) and Goldman v. United States, 316 U.S. 129 (1942), the Court had previously held the Fourth Amendment only to prevent “searches and seizures of tangible property.” Id. at 353. However, the Court noted it had been expanding this once narrow interpretation of the Fourth Amendment in recent years, recognizing a search to have occurred even when a recording of oral statements is made “without any technical trespass.” Id. Coupling this expanding view with the now clarified definition of what a reasonable expectation of privacy is, the Court determined electronically recording the conversations constituted a search and seizure under the Fourth Amendment. Id.

Under this interpretation, the government attempted to invoke several exceptions to the warrant requirement imposed by the Fourth Amendment. Id. at 356-57. However, the Court found that all the exceptions offered by the government inapplicable. Id. at 357. In fact, the Court stated “it is difficult to imagine how any of those exceptions could ever apply to the sort of search and seizure involved in this case.” Id. Instead, “electronic surveillance” is constitutionally dependent upon “the procedure of antecedent justification” in the form a search warrant. Id.