A federal agent suspected that Kyllo was growing marijuana in his home, which requires high intensity lamps. As part of his investigation, he used a thermal imaging device to see if Kyllo's condo was warmer than his neighbors' was. The scan took place from across the street, and from another street behind Kyllo's home, using a device that picks up infrared radiation. The device found that a section of his roof was warmer than the rest of his home and a lot warmer than the other homes in the complex. From this, the agent discerned (correctly) the type of lamp which Kyllo was using.
Based on this information, and on the use of informants and information from utility bills, the agents got a warrant for Kyllo's condo. They found an indoor grow with more than 100 plants, and Kyllo went to jail. He moved to suppress the marijuana, arguing that the warrant was obtained using information from an illegal search. The trial court denied his motion, so he conditionally pled guilty and appealed.
The US Supreme Court observed that searches within a home are presumptively unreasonable under the Fourth Amendment, but surveillance from outside the home does not ordinarily qualify as a search. By simply looking at a house, the police are not infringing on any reasonable expectation of privacy. The important question in this case is whether or not the use of sense enhancing equipment (like a thermal imager) converts ordinary surveillance to a search.
The Court ruled that a search has occurred when the police use a sense enhancing device which is not in general public use, and which reveals information which could not otherwise be discerned from outside the area being searched. So using a thermal imaging device is a search, since the police couldn't tell what temperature the inside of the house is unless they went into the house or used the device. Since there was no warrant and no justification for a warrantless search, this search in this case was unreasonable and the infrared evidence had to be suppressed. The case was remanded back to the trial court for a hearing on whether or not the other information used to apply for a warrant would have been sufficient to obtain one.