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Opinion of the Court
Concurring Opinion

Justice White delivered the opinion of the Court.

In United States v. Knotts, 460 U.S. 276 (1983), we held that the warrantless monitoring of an electronic tracking device ("beeper")[1] inside a container of chemicals did not violate the Fourth Amendment when it revealed no information that could not have been obtained through visual surveillance. In this case, we are called upon to address two questions left unresolved in Knotts: (1) whether installation of a beeper in a container of chemicals with the consent of the original owner constitutes a search or seizure within the meaning of the Fourth Amendment when the container is delivered to a buyer having no knowledge of the presence of the beeper, and (2) whether monitoring of a beeper falls within the ambit of the Fourth Amendment when it reveals information that could not have been obtained through visual surveillance.


In August 1980 Agent Rottinger of the Drug Enforcement Administration (DEA) learned that respondents James Karo, Richard Horton, and William Harley had ordered 50 gallons of ether from Government informant Carl Muehlenweg of Graphic Photo Design in Albuquerque, N. M. Muehlenweg told Rottinger that the ether was to be used to extract cocaine from clothing that had been imported into the United States. The Government obtained a court order authorizing the installation and monitoring of a beeper in one of the cans of ether. With Muehlenweg's consent, agents substituted their own can containing a beeper for one of the cans in the shipment and then had all 10 cans painted to give them a uniform appearance.

On September 20, 1980, agents saw Karo pick up the ether from Muehlenweg. They then followed Karo to his house using visual and beeper surveillance. At one point later that day, agents determined by using the beeper that the ether was still inside the house, but they later determined that it had been moved undetected to Horton's house, where they located it using the beeper. Agent Rottinger could smell the ether from the public sidewalk near Horton's residence. Two days later, agents discovered that the ether had once again been moved, and, using the beeper, they located it at the residence of Horton's father. The next day, the beeper was no longer transmitting from Horton's father's house, and agents traced the beeper to a commercial storage facility.

Because the beeper equipment was not sensitive enough to allow agents to learn precisely which locker the ether was in, agents obtained a subpoena for the records of the storage company and learned that locker 143 had been rented by Horton. Using the beeper, agents confirmed that the ether was indeed in one of the lockers in the row containing locker 143, and using their noses they detected the odor of ether emanating from locker 143. On October 8 agents obtained an order authorizing installation of an entry tone alarm into the door jamb of the locker so they would be able to tell when the door was opened. While installing the alarm, agents observed that the cans containing ether were still inside. Agents ceased visual and beeper surveillance, relying instead on the entry tone alarm. However, on October 16 Horton retrieved the contents from the locker without sounding the alarm. Agents did not learn of the entry until the manager of the storage facility notified them that Horton had been there.

Using the beeper, agents traced the beeper can to another self-storage facility three days later. Agents detected the smell of ether coming from locker 15 and learned from the manager that Horton and Harley had rented that locker using an alias the same day that the ether had been removed from the first storage facility. The agents obtained an order authorizing the installation of an entry tone alarm in locker 15, but instead of installing that alarm, they obtained consent from the manager of the facility to install a closed-circuit video camera in a locker that had a view of locker 15. On February 6, 1981, agents observed, by means of the video camera, Gene Rhodes and an unidentified woman removing the cans from the locker and loading them onto the rear bed of Horton's pickup truck. Using both visual and beeper surveillance agents tracked the truck to Rhodes' residence where it was parked in the driveway. Agents then observed Rhodes and a woman bringing boxes and other items from inside the house and loading the items into the trunk of an automobile. Agents did not see any cans being transferred from the pickup.

At about 6 p. m. on February 6, the car and the pickup left the driveway and traveled along public highways to Taos. During the trip, the two vehicles were under both physical and electronic surveillance. When the vehicles arrived at a house in Taos rented by Horton, Harley, and Michael Steele, the agents did not maintain tight surveillance for fear of detection. When the vehicles left the Taos residence, agents determined, using the beeper monitor, that the beeper can was still inside the house. Again on February 7, the beeper revealed that the ether can was still on the premises. At one point, agents noticed that the windows of the house were wide open on a cold windy day, leading them to suspect that the ether was being used. On February 8, the agents applied for and obtained a warrant to search the Taos residence based in part on information derived through use of the beeper. The warrant was executed on February 10, 1981, and Horton, Harley, Steele, and Evan Roth were arrested, and cocaine and laboratory equipment were seized.

Respondents Karo, Horton, Harley, Steele, and Roth were indicted for conspiring to possess cocaine with intent to distribute it and with the underlying offense. 21 U.S.C. §§ 841(a)(1) and 846. Respondent Rhodes was indicted only for conspiracy to possess. The District Court granted respondents' pretrial motion to suppress the evidence seized from the Taos residence on the grounds that the initial warrant to install the beeper was invalid and that the Taos seizure was the tainted fruit of an unauthorized installation and monitoring of that beeper. The United States appealed but did not challenge the invalidation of the initial warrant. The Court of Appeals affirmed, except with respect to Rhodes, holding that a warrant was required to install the beeper in one of the 10 cans of ether and to monitor it in private dwellings and storage lockers. 710 F.2d 1433 (CA10 1983). The warrant for the search in Taos and the resulting seizure were tainted by the prior illegal conduct of the Government. The evidence was therefore properly suppressed with respect to respondents Horton, Harley, Steele, and Roth, who were held to have protectible interests in the privacy of the Taos dwelling, and with respect to respondent Karo because the beeper had been installed without a warrant and had been monitored while its ether-can host was in his house.[2] We granted the Government's petition for certiorari, 464 U.S. 1068 (1984), which raised the question whether a warrant was required to authorize either the installation of the beeper or its subsequent monitoring. We deal with each contention in turn.


Because the judgment below in favor of Karo rested in major part on the conclusion that the installation violated his Fourth Amendment rights and that any information obtained from monitoring the beeper was tainted by the initial illegality, we must deal with the legality of the warrantless installation. It is clear that the actual placement of the beeper into the can violated no one's Fourth Amendment rights. The can into which the beeper was placed belonged at the time to the DEA, and by no stretch of the imagination could it be said that respondents then had any legitimate expectation of privacy in it. The ether and the original 10 cans, on the other hand, belonged to, and were in the possession of, Muehlenweg, who had given his consent to any invasion of those items that occurred. Thus, even if there had been no substitution of cans and the agents had placed the beeper into one of the original 10 cans, Muehlenweg's consent was sufficient to validate the placement of the beeper in the can. See United States v. Matlock, 415 U.S. 164 (1974); Frazier v. Cupp, 394 U.S. 731 (1969).

The Court of Appeals acknowledged that before Karo took control of the ether "the DEA and Muehlenweg presumably could do with the can and ether whatever they liked without violating Karo's rights." 710 F.2d, at 1438. It did not hold that the actual placement of the beeper into the ether can violated the Fourth Amendment. Instead, it held that the violation occurred at the time the beeper-laden can was transferred to Karo. The court stated:

"All individuals have a legitimate expectation of privacy that objects coming into their rightful ownership do not have electronic devices attached to them, devices that would give law enforcement agents the opportunity to monitor the location of the objects at all times and in every place that the objects are taken, including inside private residences and other areas where the right to be free from warrantless governmental intrusion is unquestioned." Ibid.

Not surprisingly, the Court of Appeals did not describe the transfer as either a "search" or a "seizure," for plainly it is neither. A "search" occurs "when an expectation of privacy that society is prepared to consider reasonable is infringed." United States v. Jacobsen, 466 U.S. 109, 113 (1984). The mere transfer to Karo of a can containing an unmonitored beeper infringed no privacy interest. It conveyed no information that Karo wished to keep private, for it conveyed no information at all. To be sure, it created a potential for an invasion of privacy, but we have never held that potential, as opposed to actual, invasions of privacy constitute searches for purposes of the Fourth Amendment. A holding to that effect would mean that a policeman walking down the street carrying a parabolic microphone capable of picking up conversations in nearby homes would be engaging in a search even if the microphone were not turned on. It is the exploitation of technological advances that implicates the Fourth Amendment, not their mere existence.

We likewise do not believe that the transfer of the container constituted a seizure. A "seizure" of property occurs when "there is some meaningful interference with an individual's possessory interests in that property." Ibid. Although the can may have contained an unknown and unwanted foreign object, it cannot be said that anyone's possessory interest was interfered with in a meaningful way. At most, there was a technical trespass on the space occupied by the beeper. The existence of a physical trespass is only marginally relevant to the question of whether the Fourth Amendment has been violated, however, for an actual trespass is neither necessary nor sufficient to establish a constitutional violation. Compare Katz v. United States, 389 U.S. 347 (1967) (no trespass, but Fourth Amendment violation), with Oliver v. United States, 466 U.S. 170 (1984) (trespass, but no Fourth Amendment violation). Of course, if the presence of a beeper in the can constituted a seizure merely because of its occupation of space, it would follow that the presence of any object, regardless of its nature, would violate the Fourth Amendment.

We conclude that no Fourth Amendment interest of Karo or of any other respondent was infringed by the installation of the beeper. Rather, any impairment of their privacy interests that may have occurred was occasioned by the monitoring of the beeper.[3]


In United States v. Knotts, 460 U.S. 276 (1983), law enforcement officials, with the consent of the seller, installed a beeper in a 5-gallon can of chloroform and monitored the beeper after delivery of the can to the buyer in Minneapolis, Minn. Although there was partial visual surveillance as the automobile containing the can moved along the public highways, the beeper enabled the officers to locate the can in the area of a cabin near Shell Lake, Wis., and it was this information that provided the basis for the issuance of a search warrant. As the case came to us, the installation of the beeper was not challenged; only the monitoring was at issue. The Court held that since the movements of the automobile and the arrival of the can containing the beeper in the area of the cabin could have been observed by the naked eye, no Fourth Amendment violation was committed by monitoring the beeper during the trip to the cabin. In Knotts, the record did not show that the beeper was monitored while the can containing it was inside the cabin, and we therefore had no occasion to consider whether a constitutional violation would have occurred had the fact been otherwise.

Here, there is no gainsaying that the beeper was used to locate the ether in a specific house in Taos, N. M., and that that information was in turn used to secure a warrant for the search of the house. The affidavit supporting the application for a search warrant recited that the ether arrived at the residence in a motor vehicle that later departed and that:

"For fear of detection, we did not maintain tight surveillance of the residence.... Using the 'beeper' locator, I positively determined that the 'beeper' can (5-gallon can of ether, described earlier in this affidavit) was now inside the above-described premises to be searched because the 'beeper' locator (direction finder) pinpointed the beeper signal as emanating from the above-described premises.... Again, later on Saturday (now in the daytime), 7 February 1981, my 'beeper' locator still shows a strong 'beeper' signal emanating from inside the above-described residence." App. 57-58.

This case thus presents the question whether the monitoring of a beeper in a private residence, a location not open to visual surveillance, violates the Fourth Amendment rights of those who have a justifiable interest in the privacy of the residence. Contrary to the submission of the United States, we think that it does.

At the risk of belaboring the obvious, private residences are places in which the individual normally expects privacy free of governmental intrusion not authorized by a warrant, and that expectation is plainly one that society is prepared to recognize as justifiable. Our cases have not deviated from this basic Fourth Amendment principle. Searches and seizures inside a home without a warrant are presumptively unreasonable absent exigent circumstances. Welsh v. Wisconsin, 466 U.S. 740, 748-749 (1984); Steagald v. United States, 451 U.S. 204, 211-212 (1981); Payton v. New York, 445 U.S. 573, 586 (1980). In this case, had a DEA agent thought it useful to enter the Taos residence to verify that the ether was actually in the house and had he done so surreptitiously and without a warrant, there is little doubt that he would have engaged in an unreasonable search within the meaning of the Fourth Amendment. For purposes of the Amendment, the result is the same where, without a warrant, the Government surreptitiously employs an electronic device to obtain information that it could not have obtained by observation from outside the curtilage of the house. The beeper tells the agent that a particular article is actually located at a particular time in the private residence and is in the possession of the person or persons whose residence is being watched. Even if visual surveillance has revealed that the article to which the beeper is attached has entered the house, the later monitoring not only verifies the officers' observations but also establishes that the article remains on the premises. Here, for example, the beeper was monitored for a significant period after the arrival of the ether in Taos and before the application for a warrant to search.

The monitoring of an electronic device such as a beeper is, of course, less intrusive than a full-scale search, but it does reveal a critical fact about the interior of the premises that the Government is extremely interested in knowing and that it could not have otherwise obtained without a warrant. The case is thus not like Knotts, for there the beeper told the authorities nothing about the interior of Knotts' cabin. The information obtained in Knotts was "voluntarily conveyed to anyone who wanted to look...," 460 U.S., at 281; here, as we have said, the monitoring indicated that the beeper was inside the house, a fact that could not have been visually verified.

We cannot accept the Government's contention that it should be completely free from the constraints of the Fourth Amendment to determine by means of an electronic device, without a warrant and without probable cause or reasonable suspicion, whether a particular article--or a person, for that matter--is in an individual's home at a particular time. Indiscriminate monitoring of property that has been withdrawn from public view would present far too serious a threat to privacy interests in the home to escape entirely some sort of Fourth Amendment oversight.[4]

We also reject the Government's contention that it should be able to monitor beepers in private residences without a warrant if there is the requisite justification in the facts for believing that a crime is being or will be committed and that monitoring the beeper wherever it goes is likely to produce evidence of criminal activity. Warrantless searches are presumptively unreasonable, though the Court has recognized a few limited exceptions to this general rule. See, e. g., United States v. Ross, 456 U.S. 798 (1982) (automobiles); Schneckloth v. Bustamonte, 412 U.S. 218 (1973) (consent); Warden v. Hayden, 387 U.S. 294 (1967) (exigent circumstances). The Government's contention that warrantless beeper searches should be deemed reasonable is based upon its deprecation of the benefits and exaggeration of the difficulties associated with procurement of a warrant. The Government argues that the traditional justifications for the warrant requirement are inapplicable in beeper cases, but to a large extent that argument is based upon the contention, rejected above, that the beeper constitutes only a minuscule intrusion on protected privacy interests. The primary reason for the warrant requirement is to interpose a "neutral and detached magistrate" between the citizen and "the officer engaged in the often competitive enterprise of ferreting out crime." Johnson v. United States, 333 U.S. 10, 14 (1948). Those suspected of drug offenses are no less entitled to that protection than those suspected of nondrug offenses. Requiring a warrant will have the salutary effect of ensuring that use of beepers is not abused, by imposing upon agents the requirement that they demonstrate in advance their justification for the desired search. This is not to say that there are no exceptions to the warrant rule, because if truly exigent circumstances exist no warrant is required under general Fourth Amendment principles.

If agents are required to obtain warrants prior to monitoring a beeper when it has been withdrawn from public view, the Government argues, for all practical purposes they will be forced to obtain warrants in every case in which they seek to use a beeper, because they have no way of knowing in advance whether the beeper will be transmitting its signals from inside private premises. The argument that a warrant requirement would oblige the Government to obtain warrants in a large number of cases is hardly a compelling argument against the requirement. It is worthy of note that, in any event, this is not a particularly attractive case in which to argue that it is impractical to obtain a warrant, since a warrant was in fact obtained in this case, seemingly on probable cause.

We are also unpersuaded by the argument that a warrant should not be required because of the difficulty in satisfying the particularity requirement of the Fourth Amendment. The Government contends that it would be impossible to describe the "place" to be searched, because the location of the place is precisely what is sought to be discovered through the search. Brief for United States 42. However true that may be, it will still be possible to describe the object into which the beeper is to be placed, the circumstances that led agents to wish to install the beeper, and the length of time for which beeper surveillance is requested. In our view, this information will suffice to permit issuance of a warrant authorizing beeper installation and surveillance.

In sum, we discern no reason for deviating from the general rule that a search of a house should be conducted pursuant to a warrant.[5]


As we have said, by maintaining the beeper the agents verified that the ether was actually located in the Taos house and that it remained there while the warrant was sought. This information was obtained without a warrant and would therefore be inadmissible at trial against those with privacy interests in the house--Horton, Harley, Steele, and Roth. That information, which was included in the warrant affidavit, would also invalidate the warrant for the search of the house if it proved to be critical to establishing probable cause for the issuance of the warrant. However, if sufficient untainted evidence was presented in the warrant affidavit to establish probable cause, the warrant was nevertheless valid. Franks v. Delaware, 438 U.S. 154, 172 (1978).

It requires only a casual examination of the warrant affidavit, which in relevant respects consists of undisputed factual assertions, to conclude that the officers could have secured the warrant without relying on the beeper to locate the ether in the house sought to be searched. The affidavit recounted the months-long tracking of the evidence, including the visual and beeper surveillance of Horton's pickup on its trip from Albuquerque to the immediate vicinity of the Taos residence; its departure a short time later without the ether; its later return to the residence; and the visual observation of the residence with its windows open on a cold night.

That leaves the question whether any part of this additional information contained in the warrant affidavit was itself the fruit of a Fourth Amendment violation to which any of the occupants of the house could object. As far as the present record reveals, two of the four respondents who had standing to object to the search of the residence--Steele and Roth-—had no interest in any of the arguably private places in which the beeper was monitored prior to its arrival in Taos. The evidence seized in the house would be admissible against them.

The question as to Horton and Harley is somewhat more complicated. On the initial leg of its journey, the ether came to rest in Karo's house where it was monitored; it then moved in succession to two other houses, including Horton's, before it was moved first to a locker in one public warehouse and then to a locker in another. Both lockers were rented jointly by Horton and Harley. On September 6, the ether was removed from the second storage facility and transported to Taos.

Assuming for present purposes that prior to its arrival at the second warehouse the beeper was illegally used to locate the ether in a house or other place in which Horton or Harley had a justifiable claim to privacy, we are confident that such use of the beeper does not taint its later use in locating the ether and tracking it to Taos. The movement of the ether from the first warehouse was undetected, but by monitoring the beeper the agents discovered that it had been moved to the second storage facility. No prior monitoring of the beeper contributed to this discovery; using the beeper for this purpose was thus untainted by any possible prior illegality. Furthermore, the beeper informed the agents only that the ether was somewhere in the warehouse; it did not identify the specific locker in which the ether was located. Monitoring the beeper revealed nothing about the contents of the locker that Horton and Harley had rented and hence was not a search of that locker.[6] The locker was identified only when agents traversing the public parts of the facility found that the smell of ether was coming from a specific locker.

The agents set up visual surveillance of that locker, and on September 6, they observed Rhodes and a female remove the ether and load it into Horton's pickup truck. The truck moved over the public streets and was tracked by beeper to Rhodes' house, where it was temporarily parked. At about 6 p. m. the truck was observed departing and was tracked visually and by beeper to the vicinity of the house in Taos. Because locating the ether in the warehouse was not an illegal search--and because the ether was seen being loaded into Horton's truck, which then traveled the public highways--it is evident that under Knotts there was no violation of the Fourth Amendment as to anyone with or without standing to complain about monitoring the beeper while it was located in Horton's truck. Under these circumstances, it is clear that the warrant affidavit, after striking the facts about monitoring the beeper while it was in the Taos residence, contained sufficient untainted information to furnish probable cause for the issuance of the search warrant. The evidence seized in the house should not have been suppressed with respect to any of the respondents.[7]

The judgment of the Court of Appeals is accordingly



^ . "A beeper is a radio transmitter, usually battery operated, which emits periodic signals that can be picked up by a radio receiver." United States v. Knotts, 460 U.S., at 277.

^ . The Court of Appeals reversed as to Rhodes since he had not shown that the beeper had been located in any place in which he had a reasonable expectation of privacy, nor had he shown any possessory interest in the ether itself that would have been invaded by the installation of the beeper.

^ . Despite this holding, warrants for the installation and monitoring of a beeper will obviously be desirable since it may be useful, even critical, to monitor the beeper to determine that it is actually located in a place not open to visual surveillance. As will be evident below, such monitoring without a warrant may violate the Fourth Amendment.

^ . Justice O'Connor observes that a homeowner has no reasonable expectation that a person invited into his home will not be wired with a microphone that transmits conversations in which he engages, see United States v. White, 401 U.S. 745 (1971), and from this proposition she concludes that a homeowner has no reasonable expectation that an invitee will not bring an object containing a beeper into his home. Post, at 722-724. While that observation would be relevant if one of the conspirators in this case had consented to the placement of the beeper in the can, it has no relevance to the case at hand. Surely if the Government surreptitiously plants a listening device on an unsuspecting household guest or family member and then monitors conversations with the homeowner, the homeowner could challenge the monitoring of the conversations regardless of the fact that he did not have power "to give effective consent to the search" of the visitor. Post, at 724. As the plurality recognized in United States v. White, supra, at 749, there is a substantial distinction between "revelation[s] to the Government by a party to conversations with the defendant" and eavesdropping on conversations without the knowledge or consent of either party to it. A homeowner takes the risk that his guest will cooperate with the Government but not the risk that a trustworthy friend has been bugged by the Government without his knowledge or consent. Under Justice O'Connor's view it could easily be said that in Katz v. United States, 389 U.S. 347 (1967), Katz had no reasonable expectation of privacy in his conversation because the person to whom he was speaking might have divulged the contents of the conversation. There would be nothing left of the Fourth Amendment right to privacy if anything that a hypothetical government informant might reveal is stripped of constitutional protection.

Rawlings v. Kentucky, 448 U.S. 98 (1980), is simply inapposite, since it was not Rawlings' home in which the challenged search occurred. Cf. Alderman v. United States, 394 U.S. 165 (1969) (homeowner has standing to challenge illegal search of house even if he has no interest in the property seized). Justice O'Connor seems to recognize as much, noting in the discussion of Katz, post, at 725, that "a third person, who never used a particular telephone line" would have no standing to challenge illegal eavesdropping. If the phone line is that of the third person, however, a different analysis is involved.

^ . The United States insists that if beeper monitoring is deemed a search, a showing of reasonable suspicion rather than probable cause should suffice for its execution. That issue, however, is not before us. The initial warrant was not invalidated for want of probable cause, which plainly existed, but for misleading statements in the affidavit. The Government did not appeal the invalidation of the warrant and as the case has turned out, the Government prevails without a warrant authorizing installation. It will be time enough to resolve the probable cause-reasonable suspicion issue in a case that requires it.

^ . Had the monitoring disclosed the presence of the container within a particular locker the result would be otherwise, for surely Horton and Harley had a reasonable expectation of privacy in their own storage locker.

^ . Although the unwarranted monitoring of the beeper in Karo's house would foreclose using that evidence against him, it did not taint the discovery of the ether in the second warehouse and the ensuing surveillance of the trip to Taos.