What key element is established in Kyllo v. United States (2001)?

The key element established in Kyllo v. United States (2001) is the limitations on thermal imaging. In this case, the Supreme Court ruled that the use of thermal imaging devices to detect heat patterns coming from a private residence constitutes a search under the Fourth Amendment. This decision reinforced the idea that individuals have a reasonable expectation of privacy in their homes, particularly against novel surveillance techniques that can reveal intimate details that would not be observable from public spaces.

The ruling emphasized that when law enforcement uses advanced technology, like thermal imaging, to gather information without a warrant, it may infringe on an individual's constitutional rights. The case set a crucial precedent regarding the legality of using such forms of surveillance and highlighted the importance of maintaining privacy against intrusive governmental practices.