Kyllo v. United States

In Kyllo v. United States, 150 L. Ed. 2d 94, federal agents suspected that Danny Kyllo was growing marijuana inside his home. Indoor marijuana growing often requires high-intensity lamps, which can generate unusual amounts of heat. To investigate, agents used a thermal-imaging device from outside the home to detect heat patterns coming from the residence.

The thermal imager did not show people or objects inside the home in the way a camera would. Instead, it measured heat being emitted from the exterior surfaces of the house. The scan showed that parts of Kyllo’s home were relatively hot compared with other areas and nearby homes. Agents treated that information as consistent with the use of indoor grow lights.

The thermal-imaging results were used as part of the basis for obtaining a search warrant. After the warrant was executed, officers found marijuana plants inside the home. Kyllo challenged the search, arguing that the use of the thermal imager was itself a Fourth Amendment search and should not have been done without a warrant.

The plain-English issue was whether police conduct a Fourth Amendment “search” when they use technology from outside a home to gather information about what is happening inside the home, without physically entering the property.

More specifically, the Court had to decide whether using a thermal imager aimed at a home, to detect heat patterns associated with activity inside, required a warrant before the scan occurred.

The Supreme Court ruled in favor of Kyllo. The Court held that the government’s use of the thermal-imaging device was a search under the Fourth Amendment.

The practical rule from Kyllo v. United States is that when officers use sense-enhancing technology to obtain information about the interior of a home that could not otherwise be obtained without a physical intrusion, and the technology is not in general public use, that use is a Fourth Amendment search. Because the home receives the highest level of Fourth Amendment protection, officers generally need a warrant before using that kind of technology to gather information about the inside of a residence.

The Court was concerned that allowing warrantless thermal imaging would create a rule that could expand as technology improves. Even if the device in this case only detected heat patterns, the constitutional question was not limited to whether the device revealed intimate details. The key point was that the government used technology not in general public use to learn information about the inside of a home that it could not otherwise have obtained without entering.

For police officers, Kyllo v. United States is a home-search technology case. If an investigative tool is used from outside a residence to learn information about conditions or activity inside the residence, officers should consider whether using that tool is itself a search requiring a warrant.

The case does not say that officers can never use thermal imagers or similar tools. It says that when the tool is used to obtain information about the interior of a home, and the information could not otherwise be obtained without physical intrusion, the Fourth Amendment is triggered if the technology is not in general public use. In that situation, officers should get a warrant before using the device as an investigative step.

The decision is especially important for investigations involving suspected indoor grow operations, hidden activity inside residences, or other situations where officers may be tempted to use technology to confirm suspicions before applying for a warrant. If the technology is being used to gather details about the inside of the home, Kyllo should be treated as a warning that the scan may need warrant authorization first.

The safer practice is to build probable cause through lawful observations, informant information, utility records where legally obtained, surveillance, and other valid investigative methods, then seek a warrant before using technology that reveals information about the home’s interior. Officers should also remember that Kyllo is tied closely to the special constitutional protection given to the home; different facts involving public spaces, vehicles, open fields, or technology in general public use may require separate legal analysis.