In Kyllo v. United States, the U.S. Supreme Court rules that the police’s use of a thermal-imaging device without a warrant to scan the outside of a house, in an effort to discover whether marijuana was growing inside, is an illegal search under the Fourth Amendment. The Court concludes that when the police “use a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion,” the homeowner retains his expectation of privacy and a warrant is needed for such a search.