The FOURTH AMENDMENT to the U.S. Constitution protects the "right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures." It further provides that "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."
Electronic surveillance did not exist in 1789, when this amendment was written and was probably not contemplated by the Founding Fathers. But the colonists were familiar with unbridled methods of law enforcement. British officials conducted warrantless SEARCHES AND SEIZURES, and made arrests based on mere suspicion. Even when a search was made pursuant to a warrant, the warrant was often general in nature, vesting British officials with absolute discretion to determine the scope and duration of the search.
The Fourth Amendment was carefully drafted in response to this colonial experience. It provides two basic protections. First, it prohibits government officials, or persons acting under color of law, from performing unreasonable searches and seizures. Second, it forbids magistrates from issuing warrants that are not supported by probable cause or that fail to specify the persons, places, and things subject to search and seizure. The Supreme Court has held that searches performed without a warrant are presumptively unreasonable. When a search is presumptively unreasonable, evidence seized by the police during the search will not be admissible against the defendant at trial unless the prosecution demonstrates that the evidence seized falls within an exception to the warrant requirement such as the "good faith" exception.
The Supreme Court first considered the Fourth Amendment implications of electronic surveillance in OLMSTEAD V. UNITED STATES, 277 U.S. 438, 48 S. Ct. 564, 72 L. Ed. 944 (1928). In Olmstead, federal agents intercepted incriminating conversations by tapping the telephone wires outside the defendant's home without a warrant or his consent. In a 5 to 4 decision, the Court ruled that electronic eavesdropping involves neither a search nor a seizure, within the meaning of the Fourth Amendment. The Court reasoned that no search took place in Olmstead because the government intercepted the conversations without entering the defendant's home or office and thus without examining any "place." No seizure occurred because the intercepted conversations were not the sort of tangible "things" the Court believed were protected by the Fourth Amendment. In a prescient dissent, Justice LOUIS D. BRANDEIS argued that nonconsensual, warrantless eavesdropping offends Fourth Amendment privacy interests without regard to manner or place of surveillance.
The Supreme Court whittled away at the Olmstead holding for the next forty years, finally overruling it in Katz v. United States, 389 U.S. 347, 88 S. Ct. 507, 19 L. Ed. 2d 576 (1967). In Katz, the police attached a listening device to the outside of a public telephone booth where the defendant was later recorded making inculpatory statements. The Court declared this type of warrantless surveillance unconstitutional. The Court emphasized that the Fourth Amendment protects persons, not places, and held that the amendment's protections extend to any place where an individual maintains a reasonable expectation of privacy. The Court determined that in Katz, the defendant maintained a reasonable expectation of privacy in both the particular conversation he had and the public telephone booth where it took place. Katz made government electronic surveillance, and legislation authorizing it, subject to the strictures of the Fourth Amendment.
As technology continues to develop, the Court has had to consider new methods of investigation by law enforcement officials. In Kyllo v. United States, 533 U.S. 27, 121 S. Ct. 2035, 150 L. Ed. 2d 94 (2001), the Court considered the constitutionality of the use of a thermal imaging device during surveillance of a home. An agent of the U.S. INTERIOR DEPARTMENT suspected that the defendant, Danny Kyllo, was growing marijuana in his home. The officer knew that indoor marijuana growth requires use of high-intensity lamps, and the officer sought to discover the presence of these lamps through the use of the thermal imaging device. The device demonstrated that the defendant was likely using a high-intensity lamp, and the agent then sought a SEARCH WARRANT from a federal magistrate judge. A subsequent search of the home discovered marijuana.
The Supreme Court, per Justice ANTONIN SCALIA, found that the use of the device to survey the inside of the defendants home constituted a "search" for Fourth Amendment purposes. The government argued that because the device only sensed heat emanating from the exterior of the house, use of the device was not an unlawful intrusion on the defendant. Scalia disagreed, noting that Katz forbids a mechanical application of the Fourth Amendment that focuses only upon the physical capability of a surveillance device. He noted, "Reversing that approach [in Katz] would leave the homeowner at the mercy of advancing technology—including imaging technology that could discern all human activity in the home." Because the agent had not obtained a warrant until after he conducted a search of Kyllo's home, the search violated Kyllo's Fourth Amendment rights.