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Kyllo v. United States
Argued February 20, 2001
Decided June 11, 2001
Full case nameDanny Lee Kyllo v. United States
Citations533 U.S. 27 (more)
121 S. Ct. 2038; 150 L. Ed. 2d 94; 2001 U.S. LEXIS 4487; 69 U.S.L.W. 4431; 2001 Cal. Daily Op. Service 4749; 2001 Daily Journal DAR 5879; 2001 Colo. J. C.A.R. 2926; 14 Fla. L. Weekly Fed. S 329
Case history
PriorUnited States v. Kyllo, 190 F.3d 1041 (9th Cir. 1999); cert. granted, 530 U.S. 1305 (2000).
Thermal imaging of a home constitutes a "search" under the Fourth Amendment and may only be done with a search warrant.
Court membership
Chief Justice
William Rehnquist
Associate Justices
John P. Stevens · Sandra Day O'Connor
Antonin Scalia · Anthony Kennedy
David Souter · Clarence Thomas
Ruth Bader Ginsburg · Stephen Breyer
Case opinions
MajorityScalia, joined by Souter, Thomas, Ginsburg, Breyer
DissentStevens, joined by Rehnquist, O'Connor, Kennedy
Laws applied
U.S. Const. amend. IV

Kyllo v. United States, 533 U.S. 27 (2001), was a decision by the Supreme Court of the United States in which the court ruled that the use of thermal imaging devices to monitor heat radiation in or around a person's home, even if conducted from a public vantage point, is unconstitutional without a search warrant.[1] In its majority opinion, the court held that thermal imaging constitutes a "search" under the Fourth Amendment, as the police were using devices to "explore details of the home that would previously have been unknowable without physical intrusion."[2] The ruling has been noted for refining the reasonable expectation of privacy doctrine in light of new surveillance technologies, and when those are used in areas that are accessible to the public.[3] This case has been praised by legal scholars since the Court refused to be the arbiter to determine "what is and is not intimate" and thus worthy of protection.[4] Instead, the Court opted to focus on "the invasiveness of the technology itself" and its ability to enable all kinds of government surveillance in the home.[4]


In 1991, federal agents with the United States Department of the Interior received a tip that Danny Lee Kyllo was growing marijuana in his home in Florence, Oregon. The agents used an FLIR thermal imaging device outside Kyllo's home, which revealed an unusual amount of heat radiating from the roof and side walls compared with the rest of the house, leading the agents to believe he was using high-intensity halide lamps that are common in cannabis cultivation.[5] The scan was conducted from a parked car across the street, without a search warrant.[6] The thermograms gathered from the scan were used by police later to obtain a warrant for a subsequent search of the house, and during that search federal agents found more than 100 marijuana plants. Kyllo was charged with one count of manufacturing marijuana.[7]

At a preliminary hearing, the court denied Kyllo's motion to suppress the evidence obtained from the thermal imaging search, ruling that the device could not "penetrate walls or windows to reveal conversations or human activities" and that the device "recorded only heat being emitted from the home".[8] Kyllo entered a conditional guilty plea before appealing to the Ninth Circuit Court of Appeals on the grounds that observations with a thermal-imaging device constituted a search under the Fourth Amendment.[9] The Ninth Circuit ultimately upheld admission of the evidence and allowed the criminal trial to continue.[8] Kyllo then petitioned the Supreme Court for a writ of certiorari, which was granted in 2000.[10]

Opinion of the Court

At the Supreme Court, the government defended the use of thermal imaging technology on two grounds: first, that thermal imaging was not an invasion of privacy, because the devices could not reveal any intimate details of the home; and second, that heat radiation can be detected in several ways from a public vantage point without the aid of technology, such as observing snow melting or smoke rising.[11]

In a majority opinion written by Justice Antonin Scalia, the Supreme Court ruled that the thermal imaging of Kyllo's home constituted a "search" within the meaning of the Fourth Amendment.[12] By a 5–4 margin, the court held that a search warrant must be obtained before the government, including the police and federal agents, may use a thermal imaging device to monitor the heat and radiation of one's home. Scalia also noted the surveillance powers that could be abused by the police with technologies that are "not in general public use".[7] As the police did not have a warrant when they used the device outside of Kyllo's home, the search was unconstitutional.[13]

Scalia wrote: "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."[14] The majority opinion argued that a person has an expectation of privacy in his or her home, and therefore, the government cannot conduct unreasonable searches, even with technology that does not enter the home.[15]

The Kyllo decision, relying on Katz v. United States (1967), confirmed the expectation of privacy in one's home, and limited the means by which the government can explore the home without a warrant. Scalia referred to the bright line drawn at the entrance of a home, where the Fourth Amendment is said to recognize a heightened expectation of privacy.[16] Furthermore, Scalia discussed how future technologies could invade one's right of privacy. Scalia argued that the Framers of the Constitution would agree that such technology would be intrusive enough to warrant a search, if they knew it existed, and as such, that technology (including thermal imaging) calls for the same warrant requirements as physical intrusion.[17]

Scalia structured the opinion to protect against more sophisticated surveillance technology that might arise in the future.[18] This was intended to protect the home from all types of warrantless surveillance and is an interpretation of what Scalia called "the long view" of the Fourth Amendment.[14]

Dissenting opinion

An example of a thermogram gathered from a thermal imaging device

In a dissenting opinion authored by Justice John Paul Stevens, the minority argued that the use of thermal imaging does not constitute a Fourth Amendment "search" because any person could detect the heat emissions. Stevens argued that this could be done simply by feeling that some areas in or around the house are warmer than others, or by observing that snow was melting more quickly on certain sections of the house. Since the public could gather this information on its own, it was therefore not unconstitutional for the government to do the same.[19] The dissenting opinion asserted that the use of the thermal imaging device was merely "off-the-wall" surveillance because it did not detect any "intimate" details within Kyllo's home.[20] According to Stevens, "Heat waves, like aromas that are generated in a kitchen, or in a laboratory or opium den, enter the public domain if and when they leave a building."[21]

Stevens especially criticized Scalia's invocation of the bright line test, calling it "unnecessary, unwise, and inconsistent with the Fourth Amendment", because this would be defunct as soon as the surveillance technology used by the police against Kyllo went into general public use.[22]

Impact and subsequent developments

Scalia's phrases "sense-enhancing technology" and "device that is not in general public use" in the Kyllo ruling have become influential in later rulings on police search procedures, but in an inconsistent fashion.[23] Several scholars and legal analysts noted the ambiguity in Scalia's use of those phrases.[18][24] Orin Kerr wrote that such phrases "can be read plausibly as suggesting a broad and even creative view of how the Fourth Amendment should respond when technology threatens privacy."[25] One journalist argued that this phrasing "sets up an unfortunate and unnecessary test that could erode our privacy as new technologies become more widespread."[26] Another legal scholar called this phrasing "vague" and "considering the rapid pace of which technology is advancing daily, troublesome to say the least";[27] while "general public use" was not adequately defined.[27]

Drone surveillance

Some analysts have questioned whether the Kyllo opinion allows the government to conduct drone surveillance on suspected criminals without a warrant, largely because drones are now commonly used by the public.[28] Raymond Nhan compared drone surveillance by law enforcement officials to the thermal imaging carried out on Danny Lee Kyllo, arguing that they can both be used to invade someone's privacy, and stressed that there's "no clear answer" on where drone technology falls in line with the Kyllo ruling.[29] Another analyst pointed out that modern drones employ infrared technology far more advanced than the scanner used on Kyllo, and drones are currently available for the general public to purchase, seemingly putting them in "general public use" according to Scalia's test.[30] Veronica McKnight argued that modern drones are now more commonplace than thermal imaging devices, and capable of surveillance that far outweighs the scans done on Kyllo.[31]

Police dog searches

A drug-detecting dog during a training exercise

Other analysts questioned whether drug-sniffing police dogs fall under the same category as thermal imaging devices under the Kyllo opinion.[5] During oral arguments in the Kyllo case, Justice Sandra Day O'Connor posed that dog sniffs would be considered unconstitutional if compared to thermal imaging, despite the Supreme Court previously upholding their constitutionality in United States v. Place (1983).[32] In the dissenting opinion in Kyllo, Stevens opined that the majority's phrasing was too broad, writing that under this new precedent, dog sniffs would be considered a search, and therefore unconstitutional without a warrant, which would effectively nullify the Place precedent.[33]

Shannon Hurley-Deal agreed that police dogs should count as sense-enhancing technology in light of Kyllo, pointing out that a dog's sense of smell is much stronger and more enhanced than that of a human.[34] Tracey Maclin agreed, writing that a drug-detecting dog, like a thermal imaging device, is a "law enforcement device that allows the police to obtain information regarding the interior of a home", and noted the conflict between the Kyllo and Place rulings.[35]

In 2005, the Supreme Court reconciled the conflict between Kyllo and Place, holding in Illinois v. Caballes that a search by a police dog was not the same as a search by thermal imaging.[36] In 2013, the Supreme Court held in Florida v. Jardines that drug-sniffing police dogs could not be brought to the front door of a person's house without a search warrant, but they did so on the basis of trespass against property rather than a right to privacy.[37] Justice Elena Kagan added in a concurring opinion that police dogs count as "devices not in general public use" as understood by the Kyllo court.[38]

See also


  1. ^ Kyllo v. United States, 533 U.S. 27 (2001).
  2. ^ "KYLLO V. UNITED STATES (99-8508) 533 U.S. 27 (2001)". Legal Information Institute. Cornell Law School. Retrieved June 4, 2022.
  3. ^ "Katz and the Adoption of the Reasonable Expectation of Privacy Test". Constitution Annotated. Library of Congress. Archived from the original on June 4, 2022. Retrieved June 4, 2022.
  4. ^ a b "Addressing the Harm of Total Surveillance: A Reply to Professor Neil Richards". Harvard Law Review. Retrieved February 22, 2024.
  5. ^ a b Froh, Amanda S. (Fall 2002). "Rethinking Canine Sniffs: The Impact of Kyllo v. United States". Seattle University Law Review. 26 (2): 337–364 – via HeinOnline.
  6. ^ Lane, Charles (February 21, 2001). "Justices Hear Oregon Case On High-Tech Surveillance". The Washington Post. Retrieved June 11, 2022.
  7. ^ a b "Fourth Amendment Search - Fuzz Shades of Gray: The New Bright-Line Rule in Determining When the Use of Technology Constitutes a Search". Wyoming Law Review. 2 (1): 169–202. 2002 – via HeinOnline.
  8. ^ a b United States v. Kyllo, 190 F.3d 1041 (9th Cir. 1999)
  9. ^ "Kyllo v. United States – Merits". United States Department of Justice. October 21, 2014. Retrieved June 13, 2022.
  10. ^ Greenhouse, Linda (September 27, 2000). "Supreme Court Adds to Its Calendar". The New York Times. p. 14. Retrieved June 13, 2022.
  11. ^ Kyllo v. United States - Oral Argument (Oral argument). February 20, 2001. Retrieved July 5, 2022 – via Oyez Project.
  12. ^ Hardee, Sarilyn E. (Fall 2001). "Why the United States Supreme Court's Rule in Kyllo v. United States Is Not the Final Word on the Constitutionality of Thermal Imaging". Campbell Law Review. 24 (1): 53–70 – via Campbell University.
  13. ^ Burling, James (February 23, 2021). "The 4th Amendment to the Constitution: A Primer". Pacific Legal Foundation. Retrieved June 13, 2022.
  14. ^ a b Kyllo v. United States, 533 U.S. at 40.
  15. ^ Anthony, Nicholas (October 28, 2021). "Why Don't Americans Have Stronger Financial Privacy Rights?". Cato Institute. Retrieved June 13, 2022. The Fourth Amendment applies to much more than just 'physical' searches.
  16. ^ "Kyllo vs. United States". Privacy Law Library. Centre for Communication Governance at National Law University Delhi. Retrieved July 11, 2022.
  17. ^ Greenhouse, Linda (July 2, 2001). "In Year of Florida Vote, Supreme Court Also Did Much Other Work". The New York Times. Retrieved June 13, 2022.
  18. ^ a b Childers, Jeffrey W. (2003). "Kyllo v. United States: A Temporary Reprieve from Technology-Enhanced Surveillance of the Home". North Carolina Law Review. 81 (2): 728–768 – via UNC School of Law.
  19. ^ Kleppetsch, Paul (Winter 2003). "In the Wake of Kyllo v. United States: The Future of Thermal Imaging Cameras". UIC Law Review. 36 (2): 517 – via University of Illinois Chicago School of Law. Justice Stevens argued that surveillance with the device failed to constitute a protected search because similar results could occur by mere use of the human senses.
  20. ^ Kyllo, 533 U.S. at 50 (Stevens, J., dissenting).
  21. ^ Kyllo, 533 U.S. at 43-44 (Stevens, J., dissenting).
  22. ^ Kyllo, 533 U.S. at 41 (Stevens, J., dissenting).
  23. ^ "FLORIDA v. JARDINES". Legal Information Institute. Cornell Law School. Retrieved June 4, 2022.
  24. ^ Friedman, Barry (January 28, 2012). "Opinion | Privacy, Technology and Law". The New York Times. Retrieved June 13, 2022.
  25. ^ Kerr, Orin S. (2004). "The Fourth Amendment and New Technologies: Constitutional Myths and the Case for Caution". Michigan Law Review. 102 (5): 831–838. doi:10.2307/4141982. JSTOR 4141982 – via JSTOR.
  26. ^ Opsahl, Kurt (October 5, 2020). "Come Back with a Warrant for my Virtual House". Electronic Frontier Foundation. Retrieved June 18, 2022.
  27. ^ a b LeFrere, J.D. (February 1, 2021). "Revisiting Kyllo v. United States' Publicly Available Technology Test and it's [sic] Implication on Drone". The Criminal Law Practitioner. Washington College of Law, American University. Retrieved June 20, 2022.
  28. ^ Feeney, Matthew (March 24, 2021). "Does the 4th Amendment Prohibit Warrantless Drone Surveillance?". Cato Institute. Retrieved June 18, 2022.
  29. ^ Nhan, Raymond (April 29, 2016). "Drones and property rights". Pacific Legal Foundation. Retrieved June 21, 2022.
  30. ^ O'Brien, Jennifer (Summer 2013). "Warrantless Government Drone Surveillance: A Challenge to the Fourth Amendment". John Marshall Journal of Information Technology and Privacy Law. 30 (1): 203–204 – via University of Illinois Chicago.
  31. ^ McKnight, Veronica E. (2015). "Drone Technology and the Fourth Amendment: Aerial Surveillance Precedent and Kyllo Do Not Account for Current Technology". California Western Law Review. 51 (2): 282–291.
  32. ^ Kyllo v. United States - Oral Argument (Oral argument). February 20, 2001. Event occurs at 16:50. Retrieved July 5, 2022 – via Oyez Project.
  33. ^ Kyllo, 533 U.S. at 47-48 (Stevens, J., dissenting).
  34. ^ Hurley-Deal, Shannon R. (2003). "State v. Fisher: Canine Sniffs - Who Let the Dogs Out". North Carolina Central Law Journal. 26 (1): 47–58.
  35. ^ Maclin, Tracey (Fall 2002). "Katz, Kyllo, and Technology: Virtual Fourth Amendment Protection in the Twenty-First Century". Mississippi Law Journal. 72 (1): 101–106 – via HeinOnline.
  36. ^ Colb, Sherry F. (November 7, 2012). "The U.S. Supreme Court Considers Dog Sniffs and the Fourth Amendment". Justia. Retrieved June 21, 2022.
  37. ^ Florida v. Jardines, 569 U.S. 1, 9 (2013).
  38. ^ Jardines, 569 U.S. at 14 (Kagan, J., concurring).

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