Court Cases;

OLR Research Report

June 25, 2001





By: Sandra Norman-Eady, Chief Attorney

You asked for a summary of Kyllo v. U.S., 533 U.S. ____ (2001), the case holding that federal agents' use of a thermal imaging camera to scan the exterior of a private home constitutes a search under the Fourth Amendment.


In a 5-4 decision penned by Justice Scalia, the U.S. Supreme Court held that the government's use of a thermal-imaging camera to determine whether someone might be using high-intensity lamps to grow marijuana plants in his home was a search within the meaning of the Fourth Amendment and thus required a warrant. Joining Justice Scalia were justices Souter, Thomas, Ginsburg, and Breyer.

The dissenting opinion, written by Justice Stevens, distinguishes between “off-the-wall” and “through-the-wall” surveillance. The dissent argued that thermal imaging involves the former, which does not violate the Fourth Amendment because it is based on observations of the exterior of a structure. “Through-the-wall” technology, which can reveal intimate details of the inside of a structure, the dissent argued, does bring the Fourth Amendment into play.

The dissent criticized the majority for announcing a general rule directed largely at yet-to-be-developed technology that might allow through-the-wall surveillance.


Facts and Proceedings Below

Federal agents used a thermal imaging device after receiving a tip that the defendant was growing marijuana in his home. The agents used the device to scan the exterior wall of the triplex where the defendant lived to determine if the amount of heat emanating from them was consistent with that generated by high-intensity lamps typically used for indoor marijuana growth.

The agents used the evidence that they obtained (i.e., pictures showing that areas of the plaintiff's home were warmer than others and the highest temperature reading was substantially warmer than other units in the triplex) plus the tip and the defendant's utility bill to get a search warrant. They found more than 100 marijuana plants in the defendant's home.

The defendant was indicted on one count of manufacturing marijuana. He entered a conditional guilty plea after he unsuccessfully argued that the evidence seized from his home should be suppressed because the thermal scan was an unconstitutional search. On appeal, the Ninth Circuit remanded the case for an evidentiary hearing on the intrusiveness of thermal imaging. The district court, finding the thermal imaging device to be non-intrusive, upheld the warrant's validity. The Court of Appeals affirmed, holding that (1) the petitioner showed no subjective expectation of privacy because he made no attempt to conceal the heat escaping from his home, and (2) there was no reasonable expectation of privacy because the imager did not expose any intimate details of the plaintiff's life. The U.S. Supreme Court granted certiorari.


Whether the use of a thermal-imaging device aimed at a private home from a public street to detect relative amounts of heat within the home constitutes a search within the meaning of the Fourth Amendment.


The Fourth Amendment provides that “[t]he right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures, shall not be violated.”


The Court applied a somewhat reverse of the principle first enunciated in Katz v. U.S., 389 U.S. 347 (1967) in assessing when a search is not a search. Katz involved the use of an electronic listening device placed on the outside of a telephone booth to eavesdrop on the conversation that took place within the booth. The Katz Court held that a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable.

The Court has subsequently applied this principle to hold that a Fourth Amendment search does not occur unless the affected person manifested a subjective expectation of privacy in the place being searched and society is willing to recognize that expectation as reasonable (California v. Ciraolo, 476 U.S. 207 (1986)).

Applying this principle to the present case, the Court found that the defendant, like all people, had at a minimum an expectation of privacy in his own home that society has acknowledged to be reasonable. To withdraw protection of this minimum expectation, the Court reasoned, would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment.

Thus, the Court found that the government's use of sense-enhancing technology to obtain information regarding the interior of the home that it could not otherwise have obtained without physical intrusion into a constitutionally protected area constitutes a search at the very least when the technology is not in general public use.

It rejected as “mechanical” the government's argument that thermal imaging should be permitted because it detects only heat radiating from the external surface of the house and does not reveal the internal contents of the home. To do otherwise, the Court concluded would leave homeowners at the mercy of advancing technology that could discern all human activity in the home. “While the technology used in the present case was relatively crude, the rule we adopt must take account of more sophisticated systems that are already in use or in development” (Slip Op., p. 8).

The Court also rejected the government's argument that thermal imaging is not a search because it does not detect private activities occurring in private areas. Relying on the Court's holding in Dow Chemical Co. v. U.S., 476 U.S. 227 (1986)), the government argued that like the aerial photography in Dow, thermal imaging did not reveal any intimate details of the plaintiff's life. In rejecting this argument, the Court distinguished the industrial complex in Dow from the surveillance of the defendant's home in the present case. According to the Court, the Fourth Amendment's protection of the home has never been tied to the quality or quantity of information obtained. The Court cited U.S. v. Karo, 468 U.S. 705 (1984) and Arizona v. Hicks, 480 U.S. 321 (1987), as examples of its long held belief that all details are intimate when they are taken from a home because the entire home is constitutionally protected from prying government eyes.

Finally, the Court found that a rule prohibiting only the use of thermal imaging to detect intimate details would be impractical because no police officer would know in advance whether his surveillance would pick up intimate details and thus would be unable to know in advance whether his action would infringe upon the target's constitutional rights.


The Court held that when the government uses a device, like the thermal imaging camera, that is not in general public use to explore details of a home that would previously have been unknowable without physical intrusion, the surveillance is a search and is presumptively unreasonable without a warrant. The Court remanded the case to the district court to decide whether there was probable cause to issue the search warrant without the evidence provided by the camera.

Dissenting Opinion

The dissent crafted a distinction between off-the-wall and through-the wall surveillances and concluded that the federal agents' use of a thermal imaging camera did not violate the Fourth Amendment. The former, according to the dissent, does not require a search warrant because its indirect deductions are based entirely on observations of the exterior of a structure.

The guiding Fourth Amendment principle, according to the dissent, is that searches and seizures of property in plain view are presumptively reasonable. Said another way, what a person exposes to the public is not the subject of Fourth Amendment protection. The thermal imaging camera in the present case passively measured heat emitted from the exterior surfaces of the defendant's home without providing details of the home's interior. The dissent argued that this observation was not protected by the Fourth Amendment, which “guarantees the right of people to be secure in their homes against unreasonable searches and seizures” (Slip Op. p.11 emphasis added).

The dissent criticized, as both too broad and too narrow, the majority's new principle that a warrant is necessary to use sense-enhancing technology to obtain information from the interior of a home, which could not otherwise have been obtained without physical intrusion into a constitutionally protected when the technology is not in the general public use. Specifically, it found the new principle's application to all sense-enhancing technology and to any information detected in the interior of a home (rather than just intimate information) to be too broad. And it found its application only to technology not in general use to be too narrow because it permits constitutional privacy guarantees to be dissipated as soon as the relevant technology is available to the general public.