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Historically, a search entailed some type of government invasion into a “constitutionally protected area.”[1] Early courts looked to the enumerated areas described in the Fourth Amendment to determine what was a “constitutionally protected area” (i.e., “persons, houses, papers, and effects”).[2] These courts soon began to emphasize property principles in their Fourth Amendment analysis.[3] Not until the landmark decision of Katz v. United States in 1967 did the U.S. Supreme Court abandon its structural “property” approach for a fluid constitutional framework that was to “protect people, not places.”[4]

Katz v. United States[]

Katz v. United States involved eavesdropping by means of an electronic surveillance device attached to the exterior of a public telephone booth — a location not within the enumerated constitutional protections (i.e., persons, houses, papers, and effects). The lower courts concluded that no search took place since the electronic surveillance device did not penetrate the wall of the telephone booth. The Supreme Court, however, stepped away from its historical property principles and proclaimed that the reach of the Fourth Amendment could not turn upon the presence or absence of a physical intrusion into a given enclosure. Although the majority in Katz demonstrated a new understanding of the term “search,” it was Justice Harlan’s concurring opinion that articulated the basic standard courts emphasize today.

According to Justice Harlan’s concurrence, a “search” does not occur for purposes of the Fourth Amendment unless (1) the individual manifested a subjective expectation of privacy in the searched object and (2) society is willing to recognize that expectation as reasonable or legitimate.[5] In essence, an impermissible “search” occurs when there is an infringement of an expectation of privacy that society is prepared to consider reasonable. Legitimate expectations of privacy must have a source outside of the Fourth Amendment, either by reference to concepts of real or personal property law or to understandings that are recognized and permitted by society.[6] Among the factors to be considered in determining whether a defendant had a legitimate expectation of privacy in addition to the proprietary or possessory interest in the place to be searched, are (1) whether the defendant has the right to exclude others from the place in question; (2) whether he has exhibited a subjective expectation of privacy that the area would remain free from governmental intrusion; and (3) whether he was legitimately on the premises.[7]

Searches vs. non-searches[]

Government surveillance where there is no reasonable expectation of privacy does not amount to a “search” within the meaning of the Fourth Amendment and therefore carries no requirement for a warrant, probable cause, or even any semblance of reasonableness. A finding that surveillance does constitute a search leads to an analysis of whether it was conducted reasonably under the circumstances. All such analysis tends to be rather fact-intensive, and factors said to be important to the analysis frequently cut against each other. The circumstances under which surveillance constitutes a search and, if so, whether it is reasonable, may depend on what information is collected from where, and how the collection is accomplished.

Katz also reinforced the “plain view” doctrine, which holds that a government official who merely observes (or smells, hears, or touches) something from a lawful vantage point does not conduct a “search.” As Justice Harlan wrote:

What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.[8]

Whether evidence can be considered to be in “plain view” of a lawfully present police officer who requires binoculars (or some other vision-enhancing technology) to view it appears to depend on whether the object is hidden and whether a court believes the equipment used to view it to be in common use, both of which are factors in assessing the legitimacy of a person’s expectation to be free from such observation.[9] That a person has taken normal precautions to maintain her privacy, that is, precautions customarily taken by those seeking to exclude others, is also a factor in determining legitimacy of expectation.[10]

Echoes of the trespass doctrine repudiated in Katz frequently reverberate throughout decisions regarding whether a given claim to an expectation of privacy is reasonable, for example, by determining whether a law enforcement officer was lawfully positioned to make a particular observation regarding the goings-on in or near a private home.[11] Consequently, persons continue to have a greater expectation of privacy in the home than they have in public places.[12] The "curtilage" of a private home[13] receives greater protection than privately owned land used for business purposes.

Open field/Curtilege doctrines[]

Under the “open field” doctrine, Fourth Amendment protection does not extend to activities that take place out of doors in an area beyond the curtilage of a home, despite efforts to maintain privacy and notwithstanding the fact that law enforcement officers had to commit trespass to come within viewing range,[14] unless perhaps particularly sophisticated sensory enhancement technology is utilized.[15]

In California v. Ciraolo,[16] the Supreme Court determined 5-4 that the aerial observation from an altitude of 1,000 feet of a fenced-in backyard within the curtilage of a home, conducted without a warrant, did not constitute a search. The defendant was growing marijuana in a small garden plot in his backyard, protected by two fences from observation by casual passers by. That the marijuana could be seen from public navigable airspace without the use of sensory enhancement equipment defeated the defendant’s claim to a reasonable expectation of privacy, even in the curtilage of his private home.[17]

On the same day that Ciraolo was handed down, the Supreme Court issued its 5-4 opinion in Dow Chemical Co. v. United States,[18] which addressed aerial photography of an industrial compound from much greater heights (but still within navigable airspace) by government regulators using a specialized mapping camera. The surveillance here was likewise not a search, although the Court suggested that such surveillance might have been a search had it involved the curtilage of a private home[19] or used less commonly available technology.[20] The Court also suggested that imagery taken from a satellite might not be permissible:

It may well be, as the Government concedes, that surveillance of private property by using highly sophisticated surveillance equipment not generally available to the public, such as satellite technology, might be constitutionally proscribed absent a warrant.[21]

The Supreme Court addressed whether an observation made from a low-flying helicopter constituted a search in Florida v. Riley,[22] a plurality concluding that it did not. At issue was the use of a police helicopter, hovering at 400 feet (an altitude prohibited for fixed-wing aircraft), to observe, through an opening in a greenhouse roof, marijuana growing inside. The plurality read Ciraolo as establishing that so long as there was no breach of the Federal Aviation Agency (FAA) safety regulations, the property owner had no legitimate reason to expect privacy with respect to non-intimate activities undertaken in the curtilage of his home that were plainly visible from above. Five justices would have preferred to consider how often members of the public actually make low-altitude helicopter flights over populated areas in determining whether the claimed expectation of privacy was reasonable. The plurality suggested that surveillance overflights that comply with FAA regulations might nevertheless constitute searches if they were to involve “undue noise, [] wind, dust, or threat of injury” or to reveal “intimate details connected with the use of the home or curtilage.”[23]

In an effort to detect and search increasingly sophisticated criminals, officials today have begun to rely more heavily on advanced technologies that seemingly intrude into our daily lives, often without our knowledge. The use of such devices may blur the line between expectations of privacy that are legitimate and those that are not. The Supreme Court addressed this issue in Kyllo v. United States[24] when it considered the constitutional limits upon the government’s use of sensory-enhancing technology. The Kyllo Court determined that the use of a thermal-imaging device to detect heat waves emitted from a home was a “search” partly because all details in the home are intimate (i.e., a person has a subjective and reasonable expectation of privacy in the interior of his home).[25] As a result, the Court held that where the government uses a device that is not in general public use, to explore details of the home or a “constitutionally protected area” that would previously have been unknowable without physical intrusion, the surveillance is a “search” and is presumptively unreasonable without a warrant.[26] The Court felt that the Fourth Amendment was to be construed in the light of what was deemed an unreasonable search and seizure when it was adopted,[27] but nonetheless, opined that it would “be foolish to contend that the degree of privacy secured to citizens by the Fourth Amendment has been entirely unaffected by the advance of technology.”[28]

The use of surveillance techniques that are so narrowly focused that they can only reveal unlawful activity or contraband may not constitute a search, at least when it takes place outside of a home and is not aimed at a person. In United States v. Place,[29] the Supreme Court concluded that the use of a drug-sniffing dog to indicate the presence of narcotics in closed luggage was not a search because it “does not expose noncontraband items that otherwise would remain hidden from public view. . . .”[30] Thus, surveillance technology that would enable the government to uncover the presence of unlawful activity or contraband, and nothing more, might not constitute a search at all. Under Kyllo, however, technology that can reveal the presence of phenomena (like heat) that may form part of a “signature” associated with unlawful activity, but not the activity itself, would nevertheless constitute a search, at least if the signature emanates from a private dwelling.[31] It is also unclear whether artificial means of limiting the information revealed by a sensor so that the operator has no way of identifying non-contraband would, by itself, make the use of such a sensor not a search.

Reasonable warrantless searches[]

If a particular type of surveillance is deemed to be a search within the meaning of the Fourth Amendment, it is permissible only if its conduct is reasonable. The “reasonableness” of a search is generally determined through a balancing test that weighs the degree to which the search intrudes upon an individual’s legitimate expectation of privacy and the degree to which it is necessary for the promotion of legitimate governmental interests.[32]

Oftentimes, the reasonableness factor may be determined by the adequacy of the applicable warrant and whether the officer conducting the search complied with its terms; however, warrants are not required in each instance. In particular, warrantless searches may be reasonable if “exigent circumstances” would prevent the timely application for a warrant.[33]

There is also a “special needs” exception for warrantless searches not based on individualized suspicion, particularly when conducted for purposes other than ordinary law enforcement.[34] For example, the government may conduct routine inspections, without warrant or suspicion, of persons and things crossing a U.S. border (or its functional equivalent), and remain within the reasonableness requirement of the Fourth Amendment.[35]

Although the Fourth Amendment does not state that its warrant requirement is limited to searches conducted in the context of criminal investigations, what is “reasonable” under those circumstances may differ from what may be deemed “reasonable” in circumstances where fewer liberty interests are arguably at stake. Many courts have found an exception to the warrant requirement for searches conducted primarily for foreign intelligence gathering purposes.[36] Evidence of criminal activity discovered during these types of permissible warrantless searches may be used in criminal prosecutions. Warrantless surveillance that falls unambiguously into a special needs exception can be conducted without a warrant subject to a test of its reasonableness, but may require a showing of probable cause in some cases.


  1. Silverman v. United States, 365 U.S. 505 (1961); Berger v. New York, 388 U.S. 41 (1967).
  2. See, e.g., Beck v. Ohio, 379 U.S. 89 (1964) (person’s clothing); Clinton v. Virginia, 377 U.S. 158 (1964) (apartment); Preston v. United States, 376 U.S. 364 (1964) (automobile interpreted as an effect).
  3. See Goldman v. United States, 316 U.S. 129, 134-35 (1942) (applying a trespass equals search analysis); Olmstead v. United States, 277 U.S. 438 (1928) (same).
  4. 389 U.S. 347, 351 (1967). See also Warden v. Hayden, 387 U.S. 294, 304 (1967) (“We have recognized that the principal object of the Fourth Amendment is the protection of privacy rather than property, and have increasingly disregarded fictional and procedural barriers rested on property concepts.”).
  5. Katz, 389 U.S. at 357 (Harlan, J., concurring).
  6. Minnesota v. Carter, 525 U.S. 83, 88 (1998).
  7. United States v. Elmore, 304 F.3d 557, 562 (6th Cir. 2002).
  8. 389 U.S. at 351-52.
  9. See Wayne R. LaFave, 1 Search and Seizure §2.2 (4th ed. 2004) (suggesting two factors: “(1) the level of sophistication of the equipment utilized by the police; and (2) the extent to which the incriminating objects or actions were out of the line of normal sight from contiguous areas where passersby or others might be”).
  10. E.g., id. at 352; United States v. Chadwick, 433 U.S. 1, 11 (1977).
  11. E.g. Harris v. United States, 390 U.S. 234 (1968).
  12. See Kyllo v. United States, 533 U.S. 27, 31 (2001) (“‘At the very core’ of the Fourth Amendment ‘stands the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion.’” (citing Silverman v. United States, 365 U.S. 505, 511 (1961)); Payton v. New York, 445 U.S. 573, 589-590 (1980).
  13. The curtilage of a dwelling is “the area to which extends the intimate activity associated with the ‘sanctity of a man’s home and the privacies of life.’” Oliver v. United States, 466 U.S. 170, 180 (1984)(quoting Boyd v. United States, 116 U.S. 616, 630 (1886)).
  14. Hester v. United States, 265 U.S. 57 (1924); Oliver v. United States, 466 U.S. 170, 177-80 (1984) (reaffirming Hester and the “open fields” doctrine in light of Katz). The majority differentiated Katz, involving the interception of a conversation from a public place, as a search of a person rather than an area. 466 U.S. at 176 & n.6.
  15. See Dow Chemical Co. v. United States, 476 U.S. 227 (1986).
  16. 476 U.S. 207 (1986).
  17. Id. at 213-14.
  18. 476 U.S. 227 (1986).
  19. Id. at 237 & n.4 (finding it “find it important that this is not an area immediately adjacent to a private home, where privacy expectations are most heightened.”) The Court also found it significant that Dow made no effort to guard against aerial surveillance. Id.
  20. Id. at 238 (“EPA was not employing some unique sensory device that, for example, could penetrate the walls of buildings and record conversations in Dow’s plants, offices, or laboratories, but rather a conventional, albeit precise, commercial camera commonly used in mapmaking.”) The dissent objected that the sophisticated and costly equipment utilized permitted the government to discern objects on the ground that a human being in an aircraft overhead could not otherwise observe. Id. at 243 & n.4 (Powell, J., dissenting).
  21. Id. at 238.
  22. 488 U.S. 445 (1989).
  23. Id. at 452 (White, J., plurality opinion).
  24. 533 U.S. 27, 34 (2001).
  25. Id. at 37.
  26. Id. at 40.
  27. Id. citing Carroll v. United States, 267 U.S. 132, 149 (1925). Accordingly, the Fourth Amendment draws a firm line at the entrance of the home against unreasonable searches and requires clear specification of those methods of surveillance that require a warrant.
  28. Kyllo, 533 U.S. at 33-34.
  29. 462 U.S. 696 (1983).
  30. Id. at 707. The Court need not have resolved the issue, having found the evidence inadmissible as the fruits of an unlawful seizure of the luggage, but the Court has followed the results in other instances. See, e.g., United States v. Jacobsen, 466 U.S. 109, 123 (1984) (chemical test that could reveal only the presence of cocaine in white powder validly in the possession of police did not infringe the defendant’s legitimate expectation of privacy because “the interest in ‘privately’ possessing cocaine [is] illegitimate”); City of Indianapolis v. Edmond, 531 U.S. 32 (2000) (canine sniffs of exteriors of automobiles stopped at random checkpoint set up to search for narcotics did not transform the temporary seizures into searches, although the seizures themselves were improper).
  31. Compare United States v. Knotts, 460 U.S. 276 (1983)(surveillance using beeper technology to track location of container on public roads does not constitute a search) with United States v. Karo, 468 U.S. 705 (1984) (use of tracking device to detect the presence of contraband within a home constitutes a search).
  32. Delaware v. Prouse, 440 U.S. 648, 654 (1979).
  33. See, e.g., Warden v. Hayden, 387 U.S. 294, 298-99 (1957) (finding warrantless search of a house justified where armed robbery suspect and weapons were believed to be inside because a delay would endanger the lives of officers and citizens).
  34. Klarfeld v. United States, 944 F.2d 583, 586 (9th Cir. 1991); see Chandler v. Miller, 520 U.S. 305, 314 (1997) (drug testing of public officials not justified by special need separate from law enforcement); Veronia School District v. Acton, 515 U.S. 646 (1995)(random drug testing of students participating in interscholastic athletics justified to deter drug use); United States v. Ramsey, 431 U.S. 606, 616 (1977) (sustaining suspicionless searches of mail entering the country); Illinois v. Andreas, 463 U.S. 765 (1983) (upholding customs searches of locked containers shipped from abroad).
  35. United States v. Flores-Montano, 541 U.S. 149, 153 (2004) (“[S]earches made at the border, pursuant to the longstanding right of the sovereign to protect itself by stopping and examining persons and property crossing into this country, are reasonable simply by virtue of the fact that they occur at the border.”) (quoting United States v. Ramsey, 431 U.S. 606, 616 (1977)).
  36. The Supreme Court held in United States v. United States District Court, 407 U.S. 297 (1972), that domestic national security surveillance by wiretapping was subject to the Warrant Clause of the Fourth Amendment, but expressly declared that its holding did not apply to electronic surveillance of foreign powers or their agents. Id. at 308. Lower courts have upheld warrantless electronic surveillance for foreign intelligence purposes. See United States v. Truong Dinh Hung, 629 F.2d 908, 912-13 (4th Cir. 1980), cert. denied, 454 U.S. 1144 (1982)(warrantless wiretap and bug of one suspected of collaborating with a foreign power held reasonable so long as the surveillance was conducted primarily for foreign intelligence reasons); United States v. Butenko, 494 F.2d 593 (3rd Cir.), cert. denied sub nom. Ivanov v. United States, 419 U.S. 881 (1974) (warrantless electronic surveillance was lawful if its primary purpose was to gather foreign intelligence information). But see Zweibon v. Mitchell, 516 F.2d 594, 613-14 (D.C. Cir. 1975), cert. denied, 425 U.S. 944 (1976) (opining in dicta that “absent exigent circumstances, all warrantless electronic surveillance is unreasonable and therefore unconstitutional”). Subsequent to these cases, Congress passed the Foreign Intelligence Surveillance Act of 1978 (FISA), P.L. 95-511, Title I, October 25, 1978, 92 Stat. 1796, codified as amended at 50 U.S.C. §1801 et seq., to provide a means for judicially authorized foreign intelligence surveillance in the United States.