Kyllo v. United States
The Court ruled that police use of a thermal-imaging camera to detect heat patterns inside a private home, without a warrant, counts as a Fourth Amendment search when the device is not something the general public commonly uses.
The decision means officers generally need a warrant before using heat-sensing or similar technology to gather information about the inside of a home, protecting homes from increasingly sophisticated surveillance tools as technology advances.
“Where, as here, the Government uses 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 surveillance is a "search" and is presumptively unreasonable without a warrant.”
The Court's core holding on when sense-enhancing technology aimed at a home counts as a search.
How it got here: After a trial court denied suppression and Kyllo pleaded guilty conditionally, the Ninth Circuit affirmed on remand, and the Supreme Court agreed to review the case.
The Case in Depth
What happened
A federal agent suspected Danny Kyllo was growing marijuana in his Oregon home using high-intensity grow lamps. At 3:20 a.m., agents parked outside and used a thermal-imaging device to scan the house from the street, discovering unusually warm areas on the roof and a wall. That scan, combined with informant tips and utility records, supported a warrant; the ensuing search found over 100 marijuana plants, and Kyllo was charged with manufacturing marijuana.
The question before the Court
Did federal agents conduct an illegal "search" when they used a thermal-imaging device to scan a man's house from the street without a warrant?
Why it matters
Police departments using thermal imagers, and eventually other advanced sensing tools, must get a warrant before pointing them at a home to learn what's happening inside. Homeowners gain protection against novel surveillance technology that reveals private activity without any officer physically entering the property.
What changes now
Because the thermal-imaging scan was an unlawful warrantless search, the case returns to the trial court, which must decide whether the search warrant for Kyllo's home was still supported by probable cause once the thermal-imaging evidence is set aside, and if not, whether any other basis exists to admit the evidence found inside the house. This is a final merits ruling, not a temporary order.
What this does not decide
The Court did not decide whether the same rule applies to devices already in widespread public use, nor did it resolve how the rule applies to more sophisticated future surveillance technology. It also left open, on remand, whether the warrant survives without the thermal-imaging evidence.
Concurrences and dissents
Dissent — Justice Stevens
“It would be quite absurd to characterize their thought processes as "searches," regardless of whether they inferred (rightly) that petitioner was growing marijuana in his house, or (wrongly) that "the lady of the house [was taking] her daily sauna and bath."”Stevens argues that drawing inferences from publicly observable heat emissions is not itself a search.
Justice Stevens argued this was simple 'off-the-wall' surveillance of heat that had already left the house into public space, no different from a neighbor noticing melting snow, so no search occurred. He would have held that only techniques giving the functional equivalent of being inside the home should count as searches, and criticized the majority's new rule as both too broad (covering harmless detection like drug-sniffing substitutes) and too narrow (limited only to homes). He warned the rule creates confusion around what counts as 'general public use.'
How the Court got there
The legal reasoning, step by step
- The Court applied the Katz test for what counts as a Fourth Amendment search — whether the government intruded on a subjective expectation of privacy that society recognizes as reasonable — but found this test hard to apply cleanly to sense-enhancing devices.
- For the interior of a home specifically, the Court identified a firmer baseline: obtaining information about the inside of a house through sense-enhancing technology that could not otherwise have been learned without physically entering a constitutionally protected area counts as a search, at least when the technology is not in common public use.
- The Court rejected the government's argument that only 'intimate details' deserve protection, reasoning that all details of what happens inside a home are treated as intimate because the whole interior is shielded from government snooping.
- The Court also rejected the idea that measuring only heat radiating off exterior walls (as opposed to seeing 'through' the walls) puts the surveillance outside Fourth Amendment concern, noting that any sense-enhancing device — a microphone, a satellite camera, or a thermal imager — only ever detects what emanates from a structure, so that distinction cannot be the dividing line.
- Applying this rule, the Court concluded that the thermal scan of Kyllo's home was a search because the device was not in general public use and revealed information about the home's interior heat patterns that officers could not have obtained without physically entering.
Doctrinal impact
Cases affected by this decision
Reaffirms Katz v. United States (389 U.S. 347)
The Court relies on Katz's expectation-of-privacy framework while extending it to sense-enhancing technology aimed at homes.
Reaffirms United States v. Karo (468 U.S. 705)
Cited as confirming that using a device to detect what's inside a home, even through inference, can be an unlawful search.
Distinguishes Dow Chemical Co. v. United States (476 U.S. 227)
The Court says aerial photography of an industrial complex doesn't apply here because homes get greater Fourth Amendment protection.