In the major case it heard on criminal procedure, the Court in Kyllo v. United States (2001), the Court held that police had to secure a warrant to use high-tech surveillance equipment. A federal agent had used a thermal imaging device to scan the building in which Danny Kyllo lived to determine whether the amount of heat emanating from the home was consistent with use of the high-intensity lamps typically required for growing marijuana indoors. The scan took only a few minutes, and showed that the roof over the garage and a side wall of the home were relatively hot compared to the rest of the building, and substantially warmer than neighboring houses. On the basis of this evidence, police secured a search warrant, found more than 100 marijuana plants inside, and arrested Kyllo. At his trial, Kyllo's attorney moved to suppress the evidence because he claimed the use of the imaging device constituted an illegal search without a warrant, but the court ruled against him.
Speaking through Justice Scalia, a 5-4 majority agreed with Kyllo that use of the thermal imaging device to detect heat use within a home was in fact a search of the home itself and thus violated the Fourth Amendment. Police were free to use the device, but only after obtaining a warrant, which meant that they had to have probable cause to believe that a particular house contained contraband. Riding up and down the street checking on houses was in effect a general warrant and forbidden. As to the minority's claim that no penetration or entry to the house had been made, Scalia responded that by using sense-enhancing technology that was not in general public use, the police had secured information regarding the interior of the home that could not otherwise have been obtained without physical intrusion into a constitutionally protected area.
The dissenters, led by Justice Stevens, believed that all the police had done was deduce information from "off-the-wall" surveillance, rather than any "through-the-wall" surveillance, and therefore the agent's conduct did not amount to a search and was perfectly reasonable. Given the growing complexity of modern technology, Stevens asserted that the courts should not erect a constitutional impediment to police use of sense-enhancing technology, but rather should let legislatures grapple with these emerging issues.
In many ways the two opinions reflect those in one of the earliest cases involving police use of technology, the original wire-tapping case of Olmstead v. United States (1928), but with the positions reversed. There the majority opinion by Chief Justice William Howard Taft took the position that since no physical entry into the house had been made a warrant was not necessary, while the dissent by Justice Louis D. Brandeis argued that regardless of whether there had been a physical intrusion, the privacy of the home had been violated. Eventually Brandeis's position became accepted as the law, and the majority opinion in this case continued that line of argument, namely, that violating the privacy of the home by any means requires a warrant by the police.