ILLINOIS v. CABALLES

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405 OCTOBER TERM, 2004
Syllabus
ILLINOIS v. CABALLES
certiorari to the supreme court of illinois
No. 03–923. Argued November 10, 2004—Decided January 24, 2005
After an Illinois state trooper stopped respondent for speeding and ra-
dioed in, a second trooper, overhearing the transmission, drove to the
scene with his narcotics-detection dog and walked the dog around re-
spondent’s car while the first trooper wrote respondent a warning ticket.
When the dog alerted at respondent’s trunk, the officers searched the
trunk, found marijuana, and arrested respondent. At respondent’s
drug trial, the court denied his motion to suppress the seized evidence,
holding, inter alia, that the dog’s alerting provided sufficient probable
cause to conduct the search. Respondent was convicted, but the Illinois
Supreme Court reversed, finding that because there were no specific
and articulable facts to suggest drug activity, use of the dog unjustifia-
bly enlarged a routine traffic stop into a drug investigation.
Held: A dog sniff conducted during a concededly lawful traffic stop that
reveals no information other than the location of a substance that no
individual has any right to possess does not violate the Fourth Amend-
ment. Pp. 407–410.
207 Ill. 2d 504, 802 N. E. 2d 202, vacated and remanded.
Stevens, J., delivered the opinion of the Court, in which O’Connor,
Scalia, Kennedy, Thomas, and Breyer, JJ., joined. Souter, J., filed a
dissenting opinion, post, p. 410. Ginsburg, J., filed a dissenting opinion,
in which Souter, J., joined, post, p. 417. Rehnquist, C. J., took no part
in the decision of the case.
Lisa Madigan, Attorney General of Illinois, argued the
cause for petitioner. With her on the briefs were Gary
Feinerman, Solicitor General, and Linda D. Woloshin and
Mary Fleming, Assistant Attorneys General.
Assistant Attorney General Wray argued the cause for
the United States as amicus curiae urging reversal. With
him on the brief were former Solicitor General Olson, Dep-
uty Solicitor General Dreeben, James A. Feldman, and John
A. Drennan.

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406 ILLINOIS v. CABALLES
Opinion of the Court
Ralph E. Meczyk argued the cause for respondent. With
him on the brief was Lawrence H. Hyman.*
Justice Stevens delivered the opinion of the Court.
Illinois State Trooper Daniel Gillette stopped respond-
ent for speeding on an interstate highway. When Gillette
radioed the police dispatcher to report the stop, a second
trooper, Craig Graham, a member of the Illinois State Police
Drug Interdiction Team, overheard the transmission and im-
mediately headed for the scene with his narcotics-detection
dog. When they arrived, respondent’s car was on the shoul-
der of the road and respondent was in Gillette’s vehicle.
While Gillette was in the process of writing a warning ticket,
Graham walked his dog around respondent’s car. The dog
alerted at the trunk. Based on that alert, the officers
searched the trunk, found marijuana, and arrested respond-
ent. The entire incident lasted less than 10 minutes.
*Briefs of amici curiae urging reversal were filed for the State of Ar-
kansas et al. by Mike Beebe, Attorney General of Arkansas, Lauren Eliza-
beth Heil, Assistant Attorney General, and Dan Schweitzer, and by the
Attorneys General for their respective States as follows: Troy King of
Alabama, Terry Goddard of Arizona, Christopher L. Morano of Connecti-
cut, M. Jane Brady of Delaware, Thurbert E. Baker of Georgia, Mark J.
Bennett of Hawaii, Lawrence G. Wasden of Idaho, Steve Carter of Indiana,
Phill Kline of Kansas, Charles C. Foti of Louisiana, G. Steven Rowe of
Maine, J. Joseph Curran, Jr., of Maryland, Michael A. Cox of Michigan,
Jon Bruning of Nebraska, Peter C. Harvey of New Jersey, Patricia A.
Madrid of New Mexico, Roy Cooper of North Carolina, Wayne Stenehjem
of North Dakota, Jim Petro of Ohio, Hardy Myers of Oregon, Henry D.
McMaster of South Carolina, Lawrence E. Long of South Dakota, Greg
Abbott of Texas, Mark L. Shurtleff of Utah, William H. Sorrell of Ver-
mont, Jerry Kilgore of Virginia, and Patrick J. Crank of Wyoming; and
for the Illinois Association of Chiefs of Police et al. by James G. Sotos.
Briefs of amici curiae urging affirmance were filed for the American
Civil Liberties Union et al. by Barry Sullivan, Jacob I. Corre´, Steven R.
Shapiro, and Harvey Grossman; and for the National Association of Crim-
inal Defense Lawyers by Jeffrey T. Green, John Wesley Hall, Jr., and
David M. Siegel.

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407 Cite as: 543 U. S. 405 (2005)
Opinion of the Court
Respondent was convicted of a narcotics offense and sen-
tenced to 12 years’ imprisonment and a $256,136 fine. The
trial judge denied his motion to suppress the seized evidence
and to quash his arrest. He held that the officers had not
unnecessarily prolonged the stop and that the dog alert was
sufficiently reliable to provide probable cause to conduct the
search. Although the Appellate Court affirmed, the Illinois
Supreme Court reversed, concluding that because the canine
sniff was performed without any “ ‘specific and articulable
facts’ ” to suggest drug activity, the use of the dog “unjusti-
fiably enlarg[ed] the scope of a routine traffic stop into a drug
investigation.” 207 Ill. 2d 504, 510, 802 N. E. 2d 202, 205
(2003).
The question on which we granted certiorari, 541 U. S. 972
(2004), is narrow: “Whether the Fourth Amendment requires
reasonable, articulable suspicion to justify using a drug-
detection dog to sniff a vehicle during a legitimate traffic
stop.” Pet. for Cert. i. Thus, we proceed on the assump-
tion that the officer conducting the dog sniff had no informa-
tion about respondent except that he had been stopped for
speeding; accordingly, we have omitted any reference to facts
about respondent that might have triggered a modicum of
suspicion.
Here, the initial seizure of respondent when he was
stopped on the highway was based on probable cause and
was concededly lawful. It is nevertheless clear that a sei-
zure that is lawful at its inception can violate the Fourth
Amendment if its manner of execution unreasonably in-
fringes interests protected by the Constitution. United
States v. Jacobsen, 466 U. S. 109, 124 (1984). A seizure that
is justified solely by the interest in issuing a warning ticket
to the driver can become unlawful if it is prolonged beyond
the time reasonably required to complete that mission. In
an earlier case involving a dog sniff that occurred during
an unreasonably prolonged traffic stop, the Illinois Supreme
Court held that use of the dog and the subsequent discovery

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408 ILLINOIS v. CABALLES
Opinion of the Court
of contraband were the product of an unconstitutional sei-
zure. People v. Cox, 202 Ill. 2d 462, 782 N. E. 2d 275 (2002).
We may assume that a similar result would be warranted in
this case if the dog sniff had been conducted while respond-
ent was being unlawfully detained.
In the state-court proceedings, however, the judges care-
fully reviewed the details of Officer Gillette’s conversations
with respondent and the precise timing of his radio transmis-
sions to the dispatcher to determine whether he had improp-
erly extended the duration of the stop to enable the dog sniff
to occur. We have not recounted those details because we
accept the state court’s conclusion that the duration of the
stop in this case was entirely justified by the traffic offense
and the ordinary inquiries incident to such a stop.
Despite this conclusion, the Illinois Supreme Court held
that the initially lawful traffic stop became an unlawful sei-
zure solely as a result of the canine sniff that occurred out-
side respondent’s stopped car. That is, the court character-
ized the dog sniff as the cause rather than the consequence
of a constitutional violation. In its view, the use of the dog
converted the citizen-police encounter from a lawful traffic
stop into a drug investigation, and because the shift in pur-
pose was not supported by any reasonable suspicion that re-
spondent possessed narcotics, it was unlawful. In our view,
conducting a dog sniff would not change the character of a
traffic stop that is lawful at its inception and otherwise exe-
cuted in a reasonable manner, unless the dog sniff itself in-
fringed respondent’s constitutionally protected interest in
privacy. Our cases hold that it did not.
Official conduct that does not “compromise any legitimate
interest in privacy” is not a search subject to the Fourth
Amendment. Jacobsen, 466 U. S., at 123. We have held
that any interest in possessing contraband cannot be deemed
“legitimate,” and thus, governmental conduct that only re-
veals the possession of contraband “compromises no legiti-
mate privacy interest.” Ibid. This is because the expecta-

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409 Cite as: 543 U. S. 405 (2005)
Opinion of the Court
tion “that certain facts will not come to the attention of the
authorities” is not the same as an interest in “privacy that
society is prepared to consider reasonable.” Id., at 122
(punctuation omitted). In United States v. Place, 462 U. S.
696 (1983), we treated a canine sniff by a well-trained
narcotics-detection dog as “sui generis” because it “discloses
only the presence or absence of narcotics, a contraband
item.” Id., at 707; see also Indianapolis v. Edmond, 531
U. S. 32, 40 (2000). Respondent likewise concedes that
“drug sniffs are designed, and if properly conducted are gen-
erally likely, to reveal only the presence of contraband.”
Brief for Respondent 17. Although respondent argues that
the error rates, particularly the existence of false positives,
call into question the premise that drug-detection dogs alert
only to contraband, the record contains no evidence or find-
ings that support his argument. Moreover, respondent does
not suggest that an erroneous alert, in and of itself, reveals
any legitimate private information, and, in this case, the trial
judge found that the dog sniff was sufficiently reliable to es-
tablish probable cause to conduct a full-blown search of the
trunk.
Accordingly, the use of a well-trained narcotics-detection
dog—one that “does not expose noncontraband items that
otherwise would remain hidden from public view,” Place, 462
U. S., at 707—during a lawful traffic stop generally does not
implicate legitimate privacy interests. In this case, the dog
sniff was performed on the exterior of respondent’s car while
he was lawfully seized for a traffic violation. Any intrusion
on respondent’s privacy expectations does not rise to the
level of a constitutionally cognizable infringement.
This conclusion is entirely consistent with our recent deci-
sion that the use of a thermal-imaging device to detect the
growth of marijuana in a home constituted an unlawful
search. Kyllo v. United States, 533 U. S. 27 (2001). Critical
to that decision was the fact that the device was capable of
detecting lawful activity—in that case, intimate details in a

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410 ILLINOIS v. CABALLES
Souter, J., dissenting
home, such as “at what hour each night the lady of the house
takes her daily sauna and bath.” Id., at 38. The legitimate
expectation that information about perfectly lawful activity
will remain private is categorically distinguishable from re-
spondent’s hopes or expectations concerning the nondetec-
tion of contraband in the trunk of his car. A dog sniff con-
ducted during a concededly lawful traffic stop that reveals
no information other than the location of a substance that
no individual has any right to possess does not violate the
Fourth Amendment.
The judgment of the Illinois Supreme Court is vacated,
and the case is remanded for further proceedings not incon-
sistent with this opinion.
It is so ordered.
The Chief Justice took no part in the decision of this
case.
Justice Souter, dissenting.
I would hold that using the dog for the purposes of deter-
mining the presence of marijuana in the car’s trunk was a
search unauthorized as an incident of the speeding stop and
unjustified on any other ground. I would accordingly affirm
the judgment of the Supreme Court of Illinois, and I respect-
fully dissent.
In United States v. Place, 462 U. S. 696 (1983), we catego-
rized the sniff of the narcotics-seeking dog as “sui generis”
under the Fourth Amendment and held it was not a search.
Id., at 707. The classification rests not only upon the limited
nature of the intrusion, but on a further premise that experi-
ence has shown to be untenable, the assumption that trained
sniffing dogs do not err. What we have learned about the
fallibility of dogs in the years since Place was decided would
itself be reason to call for reconsidering Place’s decision
against treating the intentional use of a trained dog as a
search. The portent of this very case, however, adds insist-

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411 Cite as: 543 U. S. 405 (2005)
Souter, J., dissenting
ence to the call, for an uncritical adherence to Place would
render the Fourth Amendment indifferent to suspicionless
and indiscriminate sweeps of cars in parking garages and
pedestrians on sidewalks; if a sniff is not preceded by a sei-
zure subject to Fourth Amendment notice, it escapes Fourth
Amendment review entirely unless it is treated as a search.
We should not wait for these developments to occur before
rethinking Place’s analysis, which invites such untoward
consequences.1
At the heart both of Place and the Court’s opinion today
is the proposition that sniffs by a trained dog are sui generis
because a reaction by the dog in going alert is a response to
nothing but the presence of contraband.2 See ibid. (“[T]he
sniff discloses only the presence or absence of narcotics, a
contraband item”); ante, at 409 (assuming that “a canine sniff
by a well-trained narcotics-detection dog” will only reveal
“ ‘the presence or absence of narcotics, a contraband item’ ”
(quoting Place, supra, at 707)). Hence, the argument goes,
because the sniff can only reveal the presence of items devoid
of any legal use, the sniff “does not implicate legitimate pri-
vacy interests” and is not to be treated as a search. Ante,
at 409.
The infallible dog, however, is a creature of legal fiction.
Although the Supreme Court of Illinois did not get into the
sniffing averages of drug dogs, their supposed infallibility is
belied by judicial opinions describing well-trained animals
sniffing and alerting with less than perfect accuracy, whether
1 I also join Justice Ginsburg’s dissent, post, p. 417. Without directly
reexamining the soundness of the Court’s analysis of government dog
sniffs in Place, she demonstrates that investigation into a matter beyond
the subject of the traffic stop here offends the rule in Terry v. Ohio, 392
U. S. 1 (1968), the analysis I, too, adopt.
2 Another proffered justification for sui generis status is that a dog sniff
is a particularly nonintrusive procedure. United States v. Place, 462 U. S.
696, 707 (1983). I agree with Justice Ginsburg that the introduction of
a dog to a traffic stop (let alone an encounter with someone walking down
the street) can in fact be quite intrusive. Post, at 421–422.

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412 ILLINOIS v. CABALLES
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owing to errors by their handlers, the limitations of the dogs
themselves, or even the pervasive contamination of currency
by cocaine. See, e. g., United States v. Kennedy, 131 F. 3d
1371, 1378 (CA10 1997) (describing a dog that had a 71% ac-
curacy rate); United States v. Scarborough, 128 F. 3d 1373,
1378, n. 3 (CA10 1997) (describing a dog that erroneously
alerted 4 times out of 19 while working for the postal service
and 8% of the time over its entire career); United States v.
Limares, 269 F. 3d 794, 797 (CA7 2001) (accepting as reliable
a dog that gave false positives between 7% and 38% of the
time); Laime v. State, 347 Ark. 142, 159, 60 S. W. 3d 464, 476
(2001) (speaking of a dog that made between 10 and 50 er-
rors); United States v. $242,484.00, 351 F. 3d 499, 511 (CA11
2003) (noting that because as much as 80% of all currency in
circulation contains drug residue, a dog alert “is of little
value”), vacated on other grounds by rehearing en banc, 357
F. 3d 1225 (CA11 2004); United States v. Carr, 25 F. 3d 1194,
1214–1217 (CA3 1994) (Becker, J., concurring in part and dis-
senting in part) (“[A] substantial portion of United States
currency . . . is tainted with sufficient traces of controlled
substances to cause a trained canine to alert to their pres-
ence”). Indeed, a study cited by Illinois in this case for the
proposition that dog sniffs are “generally reliable” shows
that dogs in artificial testing situations return false positives
anywhere from 12.5% to 60% of the time, depending on the
length of the search. See Reply Brief for Petitioner 13; Fed-
eral Aviation Admin., K. Garner et al., Duty Cycle of the
Detector Dog: A Baseline Study 12 (Apr. 2001) (prepared
by Auburn U. Inst. for Biological Detection Systems). In
practical terms, the evidence is clear that the dog that alerts
hundreds of times will be wrong dozens of times.
Once the dog’s fallibility is recognized, however, that ends
the justification claimed in Place for treating the sniff as sui
generis under the Fourth Amendment: the sniff alert does
not necessarily signal hidden contraband, and opening the
container or enclosed space whose emanations the dog has

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Souter, J., dissenting
sensed will not necessarily reveal contraband or any other
evidence of crime. This is not, of course, to deny that a
dog’s reaction may provide reasonable suspicion, or probable
cause, to search the container or enclosure; the Fourth
Amendment does not demand certainty of success to justify
a search for evidence or contraband. The point is simply
that the sniff and alert cannot claim the certainty that Place
assumed, both in treating the deliberate use of sniffing dogs
as sui generis and then taking that characterization as a
reason to say they are not searches subject to Fourth
Amendment scrutiny. And when that aura of uniqueness
disappears, there is no basis in Place’s reasoning, and no
good reason otherwise, to ignore the actual function that dog
sniffs perform. They are conducted to obtain information
about the contents of private spaces beyond anything that
human senses could perceive, even when conventionally en-
hanced. The information is not provided by independent
third parties beyond the reach of constitutional limitations,
but gathered by the government’s own officers in order to
justify searches of the traditional sort, which may or may
not reveal evidence of crime but will disclose anything meant
to be kept private in the area searched. Thus in practice
the government’s use of a trained narcotics dog functions as
a limited search to reveal undisclosed facts about private en-
closures, to be used to justify a further and complete search
of the enclosed area. And given the fallibility of the dog,
the sniff is the first step in a process that may disclose
“intimate details” without revealing contraband, just as a
thermal-imaging device might do, as described in Kyllo v.
United States, 533 U. S. 27 (2001).3
3 Kyllo was concerned with whether a search occurred when the police
used a thermal-imaging device on a house to detect heat emanations asso-
ciated with high-powered marijuana-growing lamps. In concluding that
using the device was a search, the Court stressed that the “Government
[may not] us[e] a device . . . to explore details of the home that would
previously have been unknowable without physical intrusion.” 533 U. S.,

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414 ILLINOIS v. CABALLES
Souter, J., dissenting
It makes sense, then, to treat a sniff as the search that it
amounts to in practice, and to rely on the body of our Fourth
Amendment cases, including Kyllo, in deciding whether such
a search is reasonable. As a general proposition, using a
dog to sniff for drugs is subject to the rule that the object
of enforcing criminal laws does not, without more, justify
suspicionless Fourth Amendment intrusions. See Indian-
apolis v. Edmond, 531 U. S. 32, 41–42 (2000). Since the
police claim to have had no particular suspicion that Caballes
was violating any drug law,4 this sniff search must stand or
fall on its being ancillary to the traffic stop that led up to it.
It is true that the police had probable cause to stop the car
for an offense committed in the officer’s presence, which
Caballes concedes could have justified his arrest. See Brief
for Respondent 31. There is no occasion to consider author-
ity incident to arrest, however, see Knowles v. Iowa, 525
U. S. 113 (1998), for the police did nothing more than detain
Caballes long enough to check his record and write a ticket.
As a consequence, the reasonableness of the search must be
assessed in relation to the actual delay the police chose to
impose, and as Justice Ginsburg points out in her opinion,
post, at 419–420, the Fourth Amendment consequences of
stopping for a traffic citation are settled law.
at 40. Any difference between the dwelling in Kyllo and the trunk of the
car here may go to the issue of the reasonableness of the respective
searches, but it has no bearing on the question of search or no search.
Nor is it significant that Kyllo’s imaging device would disclose personal
details immediately, whereas they would be revealed only in the further
step of opening the enclosed space following the dog’s alert reaction; in
practical terms the same values protected by the Fourth Amendment are
at stake in each case. The justifications required by the Fourth Amend-
ment may or may not differ as between the two practices, but if constitu-
tional scrutiny is in order for the imager, it is in order for the dog.
4 Despite the remarkable fact that the police pulled over a car for going
71 miles an hour on I–80, the State maintains that excessive speed was
the only reason for the stop, and the case comes to us on that assumption.

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Souter, J., dissenting
In Berkemer v. McCarty, 468 U. S. 420, 439–440 (1984), fol-
lowed in Knowles, supra, at 117, we held that the analogue
of the common traffic stop was the limited detention for in-
vestigation authorized by Terry v. Ohio, 392 U. S. 1 (1968).
While Terry authorized a restricted incidental search for
weapons when reasonable suspicion warrants such a safety
measure, id., at 25–26, the Court took care to keep a Terry
stop from automatically becoming a foot in the door for
all investigatory purposes; the permissible intrusion was
bounded by the justification for the detention, id., at 29–30.5
Although facts disclosed by enquiry within this limit might
give grounds to go further, the government could not other-
wise take advantage of a suspect’s immobility to search for
evidence unrelated to the reason for the detention. That
has to be the rule unless Terry is going to become an open
sesame for general searches, and that rule requires holding
that the police do not have reasonable grounds to conduct
sniff searches for drugs simply because they have stopped
someone to receive a ticket for a highway offense. Since the
police had no indication of illegal activity beyond the speed
of the car in this case, the sniff search should be held unrea-
sonable under the Fourth Amendment and its fruits should
be suppressed.
Nothing in the case relied upon by the Court, United
States v. Jacobsen, 466 U. S. 109 (1984), unsettled the limit
of reasonable enquiry adopted in Terry. In Jacobsen, the
Court found that no Fourth Amendment search occurred
when federal agents analyzed powder they had already law-
fully obtained. The Court noted that because the test could
only reveal whether the powder was cocaine, the owner had
no legitimate privacy interest at stake. 466 U. S., at 123.
5 Thus, in Place itself, the Government officials had independent grounds
to suspect that the luggage in question contained contraband before they
employed the dog sniff. 462 U. S., at 698 (describing how Place had acted
suspiciously in line at the airport and had labeled his luggage with incon-
sistent and fictional addresses).

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416 ILLINOIS v. CABALLES
Souter, J., dissenting
As already explained, however, the use of a sniffing dog in
cases like this is significantly different and properly treated
as a search that does indeed implicate Fourth Amendment
protection.
In Jacobsen, once the powder was analyzed, that was effec-
tively the end of the matter: either the powder was cocaine,
a fact the owner had no legitimate interest in concealing, or
it was not cocaine, in which case the test revealed nothing
about the powder or anything else that was not already legit-
imately obvious to the police. But in the case of the dog
sniff, the dog does not smell the disclosed contraband; it
smells a closed container. An affirmative reaction therefore
does not identify a substance the police already legitimately
possess, but informs the police instead merely of a reason-
able chance of finding contraband they have yet to put their
hands on. The police will then open the container and dis-
cover whatever lies within, be it marijuana or the owner’s
private papers. Thus, while Jacobsen could rely on the as-
sumption that the enquiry in question would either show
with certainty that a known substance was contraband or
would reveal nothing more, both the certainty and the limit
on disclosure that may follow are missing when the dog sniffs
the car.6
6 It would also be error to claim that some variant of the plain-view
doctrine excuses the lack of justification for the dog sniff in this case.
When an officer observes an object left by its owner in plain view, no
search occurs because the owner has exhibited “no intention to keep [the
object] to himself.” Katz v. United States, 389 U. S. 347, 361 (1967)
(Harlan, J., concurring). In contrast, when an individual conceals his pos-
sessions from the world, he has grounds to expect some degree of privacy.
While plain view may be enhanced somewhat by technology, see, e. g., Dow
Chemical Co. v. United States, 476 U. S. 227 (1986) (allowing for aerial
surveillance of an industrial complex), there are limits. As Kyllo v.
United States, 533 U. S. 27, 33 (2001), explained in treating the thermal-
imaging device as outside the plain-view doctrine, “[w]e have previously
reserved judgment as to how much technological enhancement of ordinary
perception” turns mere observation into a Fourth Amendment search.
While Kyllo laid special emphasis on the heightened privacy expectations

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Ginsburg, J., dissenting
The Court today does not go so far as to say explicitly that
sniff searches by dogs trained to sense contraband always
get a free pass under the Fourth Amendment, since it re-
serves judgment on the constitutional significance of sniffs
assumed to be more intrusive than a dog’s walk around a
stopped car, ante, at 409. For this reason, I do not take the
Court’s reliance on Jacobsen as actually signaling recogni-
tion of a broad authority to conduct suspicionless sniffs for
drugs in any parked car, about which Justice Ginsburg is
rightly concerned, post, at 422, or on the person of any pedes-
trian minding his own business on a sidewalk. But the
Court’s stated reasoning provides no apparent stopping point
short of such excesses. For the sake of providing a work-
able framework to analyze cases on facts like these, which
are certain to come along, I would treat the dog sniff as the
familiar search it is in fact, subject to scrutiny under the
Fourth Amendment.7
Justice Ginsburg, with whom Justice Souter joins,
dissenting.
Illinois State Police Trooper Daniel Gillette stopped Roy
Caballes for driving 71 miles per hour in a zone with a posted
that surround the home, closed car trunks are accorded some level of pri-
vacy protection. See, e. g., New York v. Belton, 453 U. S. 454, 460, n. 4
(1981) (holding that even a search incident to arrest in a vehicle does not
itself permit a search of the trunk). As a result, if Fourth Amendment
protections are to have meaning in the face of superhuman, yet fallible,
techniques like the use of trained dogs, those techniques must be justified
on the basis of their reasonableness, lest everything be deemed in plain
view.
7 I should take care myself to reserve judgment about a possible case
significantly unlike this one. All of us are concerned not to prejudge a
claim of authority to detect explosives and dangerous chemical or biologi-
cal weapons that might be carried by a terrorist who prompts no individu-
alized suspicion. Suffice it to say here that what is a reasonable search
depends in part on demonstrated risk. Unreasonable sniff searches for
marijuana are not necessarily unreasonable sniff searches for destructive
or deadly material if suicide bombs are a societal risk.

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418 ILLINOIS v. CABALLES
Ginsburg, J., dissenting
speed limit of 65 miles per hour. Trooper Craig Graham of
the Drug Interdiction Team heard on the radio that Trooper
Gillette was making a traffic stop. Although Gillette re-
quested no aid, Graham decided to come to the scene to
conduct a dog sniff. Gillette informed Caballes that he was
speeding and asked for the usual documents—driver’s li-
cense, car registration, and proof of insurance. Caballes
promptly provided the requested documents but refused to
consent to a search of his vehicle. After calling his dis-
patcher to check on the validity of Caballes’ license and for
outstanding warrants, Gillette returned to his vehicle to
write Caballes a warning ticket. Interrupted by a radio call
on an unrelated matter, Gillette was still writing the ticket
when Trooper Graham arrived with his drug-detection dog.
Graham walked the dog around the car, the dog alerted at
Caballes’ trunk, and, after opening the trunk, the troopers
found marijuana. 207 Ill. 2d 504, 506–507, 802 N. E. 2d 202,
203 (2003).
The Supreme Court of Illinois held that the drug evidence
should have been suppressed. Id., at 506, 802 N. E. 2d, at
202. Adhering to its decision in People v. Cox, 202 Ill. 2d
462, 782 N. E. 2d 275 (2002), the court employed a two-part
test taken from Terry v. Ohio, 392 U. S. 1 (1968), to deter-
mine the overall reasonableness of the stop. 207 Ill. 2d, at
508, 802 N. E. 2d, at 204. The court asked first “whether
the officer’s action was justified at its inception,” and second
“whether it was reasonably related in scope to the circum-
stances which justified the interference in the first place.”
Ibid. (quoting People v. Brownlee, 186 Ill. 2d 501, 518–519,
713 N. E. 2d 556, 565 (1999) (in turn quoting Terry, 392 U. S.,
at 19–20)). “[I]t is undisputed,” the court observed, “that
the traffic stop was properly initiated”; thus, the dispositive
inquiry trained on the “second part of the Terry test,” in
which “[t]he State bears the burden of establishing that the
conduct remained within the scope of the stop.” 207 Ill. 2d,
at 509, 802 N. E. 2d, at 204.

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419 Cite as: 543 U. S. 405 (2005)
Ginsburg, J., dissenting
The court concluded that the State failed to offer sufficient
justification for the canine sniff: “The police did not detect
the odor of marijuana in the car or note any other evidence
suggesting the presence of illegal drugs.” Ibid. Lacking
“specific and articulable facts” supporting the canine sniff,
ibid. (quoting Cox, 202 Ill. 2d, at 470–471, 782 N. E. 2d, at
281), the court ruled, “the police impermissibly broadened
the scope of the traffic stop in this case into a drug investiga-
tion.” 207 Ill. 2d, at 509, 802 N. E. 2d, at 204.1 I would
affirm the Illinois Supreme Court’s judgment and hold that
the drug sniff violated the Fourth Amendment.
In Terry v. Ohio, the Court upheld the stop and subse-
quent frisk of an individual based on an officer’s observation
of suspicious behavior and his reasonable belief that the sus-
pect was armed. See 392 U. S., at 27–28. In a Terry-type
investigatory stop, “the officer’s action [must be] justified at
its inception, and . . . reasonably related in scope to the cir-
cumstances which justified the interference in the first
place.” Id., at 20. In applying Terry, the Court has several
times indicated that the limitation on “scope” is not confined
to the duration of the seizure; it also encompasses the man-
ner in which the seizure is conducted. See, e. g., Hiibel v.
Sixth Judicial Dist. Court of Nev., Humboldt Cty., 542 U. S.
177, 188 (2004) (an officer’s request that an individual identify
himself “has an immediate relation to the purpose, rationale,
and practical demands of a Terry stop”); United States v.
Hensley, 469 U. S. 221, 235 (1985) (examining, under Terry,
1 The Illinois Supreme Court held insufficient to support a canine sniff
Gillette’s observations that (1) Caballes said he was moving to Chicago,
but his only visible belongings were two sport coats in the backseat; (2) the
car smelled of air freshener; (3) Caballes was dressed for business,
but was unemployed; and (4) Caballes seemed nervous. Even viewed
together, the court said, these observations gave rise to “nothing more
than a vague hunch” of “possible wrongdoing.” 207 Ill. 2d 504, 509–510,
802 N. E. 2d 202, 204–205 (2003). This Court proceeds on “the assump-
tion that the officer conducting the dog sniff had no information about
[Caballes].” Ante, at 407.

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420 ILLINOIS v. CABALLES
Ginsburg, J., dissenting
both “the length and intrusiveness of the stop and deten-
tion”); Florida v. Royer, 460 U. S. 491, 500 (1983) (plurality
opinion) (“[A]n investigative detention must be temporary
and last no longer than is necessary to effectuate the purpose
of the stop [and] the investigative methods employed should
be the least intrusive means reasonably available to verify
or dispel the officer’s suspicion . . . .”).
“A routine traffic stop,” the Court has observed, “is a rela-
tively brief encounter and ‘is more analogous to a so-called
Terry stop . . . than to a formal arrest.’ ” Knowles v. Iowa,
525 U. S. 113, 117 (1998) (quoting Berkemer v. McCarty, 468
U. S. 420, 439 (1984)); see also ante, at 415 (Souter, J.,
dissenting) (The government may not “take advantage of
a suspect’s immobility to search for evidence unrelated to
the reason for the detention.”).2 I would apply Terry’s
reasonable-relation test, as the Illinois Supreme Court did,
to determine whether the canine sniff impermissibly ex-
panded the scope of the initially valid seizure of Caballes.
It is hardly dispositive that the dog sniff in this case may
not have lengthened the duration of the stop. Cf. ante, at
407 (“A seizure . . . can become unlawful if it is prolonged
beyond the time reasonably required to complete [the initial]
mission.”). Terry, it merits repetition, instructs that any
investigation must be “reasonably related in scope to the
circumstances which justified the interference in the first
place.” 392 U. S., at 20 (emphasis added). The unwar-
2 The Berkemer Court cautioned that by analogizing a traffic stop to a
Terry stop, it did “not suggest that a traffic stop supported by probable
cause may not exceed the bounds set by the Fourth Amendment on the
scope of a Terry stop.” 468 U. S., at 439, n. 29. This Court, however,
looked to Terry earlier in deciding that an officer acted reasonably when
he ordered a motorist stopped for driving with expired license tags to exit
his car, Pennsylvania v. Mimms, 434 U. S. 106, 109–110 (1977) (per cu-
riam), and later reaffirmed the Terry analogy when evaluating a po-
lice officer’s authority to search a vehicle during a routine traffic stop,
Knowles, 525 U. S., at 117.

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421 Cite as: 543 U. S. 405 (2005)
Ginsburg, J., dissenting
ranted and nonconsensual expansion of the seizure here from
a routine traffic stop to a drug investigation broadened the
scope of the investigation in a manner that, in my judgment,
runs afoul of the Fourth Amendment.3
The Court rejects the Illinois Supreme Court’s judgment
and, implicitly, the application of Terry to a traffic stop con-
verted, by calling in a dog, to a drug search. The Court so
rules, holding that a dog sniff does not render a seizure that
is reasonable in time unreasonable in scope. Ante, at 408.
Dog sniffs that detect only the possession of contraband may
be employed without offense to the Fourth Amendment, the
Court reasons, because they reveal no lawful activity and
hence disturb no legitimate expectation of privacy. Ante,
at 408–409.
In my view, the Court diminishes the Fourth Amendment’s
force by abandoning the second Terry inquiry (was the police
action “reasonably related in scope to the circumstances [jus-
tifiying] the [initial] interference”). 392 U. S., at 20. A
drug-detection dog is an intimidating animal. Cf. United
States v. Williams, 356 F. 3d 1268, 1276 (CA10 2004) (McKay,
J., dissenting) (“drug dogs are not lap dogs”). Injecting such
an animal into a routine traffic stop changes the character of
the encounter between the police and the motorist. The
stop becomes broader, more adversarial, and (in at least
some cases) longer. Caballes—who, as far as Troopers
Gillette and Graham knew, was guilty solely of driving six
miles per hour over the speed limit—was exposed to the em-
barrassment and intimidation of being investigated, on a
public thoroughfare, for drugs. Even if the drug sniff is not
characterized as a Fourth Amendment “search,” cf. Indian-
3 The question whether a police officer inquiring about drugs without
reasonable suspicion unconstitutionally broadens a traffic investigation is
not before the Court. Cf. Florida v. Bostick, 501 U. S. 429, 434 (1991)
(police questioning of a bus passenger, who might have just said “No,” did
not constitute a seizure).

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422 ILLINOIS v. CABALLES
Ginsburg, J., dissenting
apolis v. Edmond, 531 U. S. 32, 40 (2000); United States v.
Place, 462 U. S. 696, 707 (1983), the sniff surely broadened
the scope of the traffic-violation-related seizure.
The Court has never removed police action from Fourth
Amendment control on the ground that the action is well
calculated to apprehend the guilty. See, e. g., United States
v. Karo, 468 U. S. 705, 717 (1984) (Fourth Amendment war-
rant requirement applies to police monitoring of a beeper in
a house even if “the facts [justify] 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.”); see also Minnesota v. Carter, 525 U. S. 83, 110
(1998) (Ginsburg, J., dissenting) (“Fourth Amendment pro-
tection, reserved for the innocent only, would have little
force in regulating police behavior toward either the inno-
cent or the guilty.”). Under today’s decision, every traffic
stop could become an occasion to call in the dogs, to the dis-
tress and embarrassment of the law-abiding population.
The Illinois Supreme Court, it seems to me, correctly
apprehended the danger in allowing the police to search for
contraband despite the absence of cause to suspect its pres-
ence. Today’s decision, in contrast, clears the way for suspi-
cionless, dog-accompanied drug sweeps of parked cars along
sidewalks and in parking lots. Compare, e. g., United States
v. Ludwig, 10 F. 3d 1523, 1526–1527 (CA10 1993) (upholding
a search based on a canine drug sniff of a parked car in a
motel parking lot conducted without particular suspicion),
with United States v. Quinn, 815 F. 2d 153, 159 (CA1 1987)
(officers must have reasonable suspicion that a car contains
narcotics at the moment a dog sniff is performed), and Place,
462 U. S., at 706–707 (Fourth Amendment not violated by a
dog sniff of a piece of luggage that was seized, pre-sniff,
based on suspicion of drugs). Nor would motorists have
constitutional grounds for complaint should police with dogs,
stationed at long traffic lights, circle cars waiting for the red
signal to turn green.

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423 Cite as: 543 U. S. 405 (2005)
Ginsburg, J., dissenting
Today’s decision also undermines this Court’s situation-
sensitive balancing of Fourth Amendment interests in other
contexts. For example, in Bond v. United States, 529 U. S.
334, 338–339 (2000), the Court held that a bus passenger had
an expectation of privacy in a bag placed in an overhead
bin and that a police officer’s physical manipulation of the
bag constituted an illegal search. If canine drug sniffs are
entirely exempt from Fourth Amendment inspection, a sniff
could substitute for an officer’s request to a bus passenger
for permission to search his bag, with this significant differ-
ence: The passenger would not have the option to say “No.”
The dog sniff in this case, it bears emphasis, was for drug
detection only. A dog sniff for explosives, involving security
interests not presented here, would be an entirely differ-
ent matter. Detector dogs are ordinarily trained not as
all-purpose sniffers, but for discrete purposes. For exam-
ple, they may be trained for narcotics detection or for explo-
sives detection or for agricultural products detection. See,
e. g., U. S. Customs & Border Protection, Canine Enforce-
ment Training Center Training Program Course Descrip-
tions, http://www.cbp.gov/xp/cgov/ border_security/canines/
training_program.xml (all Internet materials as visited Dec.
16, 2004, and available in Clerk of Court’s case file) (de-
scribing Customs training courses in narcotics detection);
Transportation Security Administration, Canine and Explo-
sives Program, http://www.tsa.gov/public/display?theme=32
(describing Transportation Security Administration’s explo-
sives detection canine program); U. S. Dept. of Agriculture,
Animal and Plant Health Inspection Service, USDA’s Detec-
tor Dogs: Protecting American Agriculture (Oct. 2001), avail-
able at http://www.aphis.usda.gov/oa/pubs/detdogs.pdf (de-
scribing USDA Beagle Brigade detector dogs trained to
detect prohibited fruits, plants, and meat); see also Jennings,
Origins and History of Security and Detector Dogs, in Ca-
nine Sports Medicine and Surgery 16, 18–19 (M. Bloomberg,
J. Dee, & R. Taylor eds. 1998) (describing narcotics-detector

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424 ILLINOIS v. CABALLES
Ginsburg, J., dissenting
dogs used by Border Patrol and Customs, and bomb detec-
tor dogs used by the Federal Aviation Administration and
the Secret Service, but noting the possibility in some cir-
cumstances of cross training dogs for multiple tasks); S.
Chapman, Police Dogs in North America 64, 70–79 (1990) (de-
scribing narcotics- and explosives-detection dogs and noting
the possibility of cross training). There is no indication in
this case that the dog accompanying Trooper Graham was
trained for anything other than drug detection. See 207 Ill.
2d, at 507, 802 N. E. 2d, at 203 (“Trooper Graham arrived
with his drug-detection dog . . . .”); Brief for Petitioner 3
(“Trooper Graham arrived with a drug-detection dog . . . .”).
This Court has distinguished between the general interest
in crime control and more immediate threats to public safety.
In Michigan Dept. of State Police v. Sitz, 496 U. S. 444
(1990), this Court upheld the use of a sobriety traffic check-
point. Balancing the State’s interest in preventing drunk
driving, the extent to which that could be accomplished
through the checkpoint program, and the degree of intrusion
the stops involved, the Court determined that the State’s
checkpoint program was consistent with the Fourth Amend-
ment. Id., at 455. Ten years after Sitz, in Indianapolis v.
Edmond, 531 U. S. 32, this Court held that a drug interdic-
tion checkpoint violated the Fourth Amendment. Despite
the illegal narcotics traffic that the Nation is struggling to
stem, the Court explained, a “general interest in crime con-
trol” did not justify the stops. Id., at 43–44 (internal quota-
tion marks omitted). The Court distinguished the sobriety
checkpoints in Sitz on the ground that those checkpoints
were designed to eliminate an “immediate, vehicle-bound
threat to life and limb.” 531 U. S., at 43.
The use of bomb-detection dogs to check vehicles for ex-
plosives without doubt has a closer kinship to the sobriety
checkpoints in Sitz than to the drug checkpoints in Edmond.
As the Court observed in Edmond: “[T]he Fourth Amend-
ment would almost certainly permit an appropriately tai-

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425 Cite as: 543 U. S. 405 (2005)
Ginsburg, J., dissenting
lored roadblock set up to thwart an imminent terrorist
attack . . . .” 531 U. S., at 44. Even if the Court were to
change course and characterize a dog sniff as an independent
Fourth Amendment search, see ante, p. 410 (Souter, J., dis-
senting), the immediate, present danger of explosives would
likely justify a bomb sniff under the special needs doctrine.
See, e. g., ante, at 417, n. 7 (Souter, J., dissenting); Griffin
v. Wisconsin, 483 U. S. 868, 873 (1987) (permitting exceptions
to the warrant and probable-cause requirements for a search
when “special needs, beyond the normal need for law en-
forcement,” make those requirements impracticable (quoting
New Jersey v. T. L. O., 469 U. S. 325, 351 (1985) (Blackmun,
J., concurring in judgment))).
* * *
For the reasons stated, I would hold that the police vio-
lated Caballes’ Fourth Amendment rights when, without
cause to suspect wrongdoing, they conducted a dog sniff of
his vehicle. I would therefore affirm the judgment of the
Illinois Supreme Court.