                           PUBLISHED

UNITED STATES COURT OF APPEALS
                FOR THE FOURTH CIRCUIT


UNITED STATES OF AMERICA,               
                 Plaintiff-Appellant,
                 v.
                                                No. 03-4715
HARSIMRAT SINGH; RANDHIR SINGH
KHANGURA,
             Defendants-Appellees.
                                        
            Appeal from the United States District Court
       for the Middle District of North Carolina, at Durham.
                James A. Beaty, Jr., District Judge.
                            (CR-03-94)

                      Argued: January 23, 2004

                       Decided: April 2, 2004

    Before WILLIAMS, TRAXLER, and KING, Circuit Judges.



Reversed and remanded by published opinion. Judge King wrote the
opinion, in which Judge Williams and Judge Traxler joined.


                            COUNSEL

ARGUED: Kearns Davis, Assistant United States Attorney, Greens-
boro, North Carolina, for Appellant. William Lindsay Osteen, Jr.,
ADAMS & OSTEEN, Greensboro, North Carolina, for Appellee Har-
simrat Singh; James Daniel Williams, Jr., Durham, North Carolina,
for Appellee Randhir Singh Khangura. ON BRIEF: Anna Mills Wag-
oner, United States Attorney, Sandra J. Hairston, Assistant United
States Attorney, Greensboro, North Carolina, for Appellant.
2                         UNITED STATES v. SINGH
                                OPINION

KING, Circuit Judge:

   Harsimrat Singh and Randhir Singh Khangura were indicted on
March 31, 2003, in the Middle District of North Carolina for posses-
sion with intent to distribute marijuana. Singh and Khangura thereaf-
ter sought to suppress a large quantity of marijuana and currency
seized from the trailer of their tractor-trailer rig. The district court
heard evidence on the suppression issues and granted the relief
requested. United States v. Singh, No. 1:03CR94-1 (M.D.N.C. Aug.
28, 2003) (the "Opinion"). The Government has appealed, maintain-
ing that the court erred in ruling the seizure unconstitutional. Because
the seizure was supported by probable cause and passes constitutional
muster, we reverse and remand.

                                      I.

                                     A.

   The facts underlying this dispute, as presented to the district court
at an evidentiary hearing in Winston-Salem on July 28 and 29, 2003,
are largely uncontroverted.1 On March 8, 2003, federal agent Eric
Green learned from a confidential informant in Atlanta, Georgia (the
"CI"), that a tractor-trailer rig (the "Rig") containing a large quantity
of marijuana had broken down on Interstate 85 ("I-85") south at mile-
post 201 or 202 near Greensboro, North Carolina. The CI further
advised Green that the disabled Rig’s tractor was silver in color, that
its tractor and trailer bore Canadian license plates, and that it was
being driven from Canada to Atlanta by "two Indian males" who were
awaiting tow truck assistance. Opinion at 2. Green relayed this infor-
mation to one of his colleagues in the Bureau of Immigration and
Customs Enforcement ("BICE"), agent James Bryant, in Charlotte,
North Carolina. Bryant, in an effort to locate and intercept the smug-
gled contraband, relayed this information to the North Carolina High-
way Patrol (the "NCHP"). J.A. 69, 71.
    1
   In laying out the relevant facts, we are mindful of our obligation to
defer to the findings of the district court in the absence of clear error. See
Ornelas v. United States, 517 U.S. 690, 699 (1996).
                        UNITED STATES v. SINGH                          3
   After determining that mileposts 201 and 202 on I-85 were not in
the Greensboro area, the NCHP attempted to locate the Rig at mile-
post 101 on I-85 and at milepost 202 on Interstate 40, near Greens-
boro. Failing to locate the disabled Rig at these locations, NCHP
officers recontacted agent Bryant to seek clarification on the location
of the Rig. Bryant, in turn, contacted agent Green in Atlanta, request-
ing such information. J.A. 92. As a result, the CI phoned the truckers,
who were with the disabled Rig in North Carolina, and obtained clari-
fication on their location. Through these communications, the CI, in
agent Green’s presence, ascertained from the truckers that the dis-
abled Rig was near milepost 202 on I-85, approximately seventy
miles north of Greensboro and just south of the North Carolina-
Virginia line. This information was then relayed to the NCHP. Opin-
ion at 2-3. (The information provided by the CI is hereinafter referred
to as "the Tip.")

   Upon determining that milepost 202 on I-85 was located in North
Carolina’s Granville County, NCHP Sergeant Bill Grey directed an
officer to go there, and he confirmed the presence of a disabled rig
matching the description provided by the CI. When that information
was relayed to Grey, three NCHP officers — Grey, Terry Siler, and
Greg Strader — departed the Greensboro area in separate cruisers,
proceeding north seventy to eighty miles on I-85 to Granville County.
Contemporaneously, Grey contacted an NCHP K-9 narcotic detection
unit and requested that it proceed to milepost 202 to assist in locating
contraband.2 Id. at 3.

   Upon reaching milepost 202, the officers located the disabled Rig.
By that time, the trailer of the Rig had been dismounted from its trac-
tor, and the tractor and trailer were attached to separate tow trucks.
Because the officers were unable to cross the I-85 median, they pro-
ceeded north to Exit 204 before turning southbound. Strader then pro-
ceeded to the site where they had observed the disabled Rig, and Grey
and Siler followed shortly thereafter. When Strader arrived at mile-
post 202, the tow trucks had departed, towing the Rig south on I-85.
  2
   Sergeant Grey and Trooper Strader are part of the NCHP Criminal
Interdiction Team, whose members have received additional training in
criminal interdiction skills. The K-9 narcotic detection unit is also part
of the Team. Opinion at 2; J.A. 199.
4                       UNITED STATES v. SINGH
Consequently, Strader, Grey, and Siler continued south on I-85 in
search of the Rig. Id. at 3-4.

   Upon locating the Rig, Strader observed that the lead tow truck was
towing the Rig’s trailer and that the other tow truck, with the Rig’s
tractor, was following closely behind. Opinion at 4. He also observed
that the Rig’s tractor and trailer each bore Canadian license plates.
J.A. 167. Using his speed detection equipment, Strader determined
that the lead tow truck was travelling seventy-one miles per hour in
a sixty-five mile-per-hour work zone, and he stopped it at I-85’s mile-
post 180, near Oxford, in Durham County. J.A. 152. The second tow
truck then also stopped, pulling onto the shoulder of I-85 in front of
the lead tow truck and Strader’s cruiser. Opinion at 5.

   The driver of the speeding tow truck, Erskine Williamson, exited
to speak with Strader, and the two men met at the front of the truck.
As Strader was explaining the speeding incident to Williamson, they
were joined by Michael Huff, the driver of the second tow truck. Soon
thereafter, Grey joined the conversation, and Strader asked William-
son whether the Rig had been in an accident or had broken down.
Williamson explained that the Rig had broken down, that one of the
Rig’s occupants was the passenger in his tow truck, and that the other
trucker was in Huff’s truck. Williamson advised the officers that the
truckers "were anxious to have their tractor repaired as soon as possi-
ble, no matter what the price," and that they wanted the tractor "towed
to the repair shop where Williamson and Huff worked but wanted the
trailer towed to a different location so it could be picked up." Id. at
6. Huff advised the officers that the trucker in his tow truck had
become "visibly nervous or agitated" upon seeing Strader pull along-
side. Id. Indeed, according to Huff, after seeing Strader signal for
Williamson to stop, the trucker in Huff’s tow truck "almost jumped
out of his skin" and immediately attempted to place a call on his cell
phone. J.A. 207. Strader, Williamson, and Huff viewed this behavior
as both "strange" and "unusual."3 J.A. 125, 148, 151, 309.
   3
     Williamson and Huff testified that they advised Strader and Grey
about Singh’s and Khangura’s behavior after Strader issued Williamson
a warning citation for speeding. Strader and Grey, on the other hand, tes-
tified that Williamson and Huff discussed Singh’s and Khangura’s
behavior prior to issuance of the citation. Opinion at 6-7. This apparent
discrepancy is irrelevant to our assessment.
                         UNITED STATES v. SINGH                          5
   Strader then directed Khangura to exit Williamson’s tow truck.
After joining the two officers, Khangura was identified, and he was
questioned by Strader regarding the Rig and its contents. Strader
ascertained that Khangura and his co-driver were not American citi-
zens, but were "from another country." J.A. 180-81. Khangura
explained that he and his co-driver were travelling from Boston to
Charlotte and that they were running empty, hoping to arrange for a
load in Charlotte. Opinion at 7-8. Grey and Strader considered these
responses suspicious, doubting that such a long journey would be
undertaken with no guarantee of remuneration. J.A. 148, 207. Strader
then returned to his cruiser with Williamson and Grey and wrote Wil-
liamson a warning citation for speeding. Meanwhile, Huff and
Khangura remained near the front of Williamson’s truck. Opinion at
8.

    After Strader prepared the citation, Williamson was advised to
"stand by" while the officers spoke with Singh and Khangura. Grey
and Williamson then rejoined Huff and Khangura. Strader, however,
went past the group to Huff’s tow truck and requested Singh to exit
it. Singh was questioned briefly, and he generally provided the same
responses Strader had received from Khangura. Additionally, Singh
explained that he had worked for Yadu Transport, the company
named on the trailer, for three weeks. Khangura stated that he had
worked for Yadu for only one week. Singh and Khangura then pro-
vided conflicting information regarding the number of rigs in Yadu’s
truck fleet. They were then asked if they had marijuana, cocaine, or
guns in the tractor or the trailer, and both responded in the negative.
In light of the Tip and the corroborating circumstances, the officers
requested permission to search the Rig, and at least one of the two
men orally consented. The officers then presented Singh and
Khangura with a written consent-to-search form, memorializing the
verbal consent, which they both executed.4 Id. at 8-9.

   Shortly after Singh and Khangura signed the consent-to-search
form, the NCHP K-9 unit, which had been requested to stand by,
arrived and used its drug detection dog "Rico" to sweep the outside
  4
   The consent-to-search form signed by Singh and Khangura consented
to a complete search of a gray Peterbilt TTT (truck, tractor, and trailer),
the tractor of which bore Ontario, Canada, license PT 2768. J.A. 33.
6                         UNITED STATES v. SINGH
of the Rig’s tractor and trailer. Rico alerted to the "odor of narcotics"
at the left side of the front of the trailer and at the trailer’s rear doors.5
Id. at 9. The officers then opened the unlocked doors of the Rig’s
trailer and found that two plywood panels lining the front wall
"seemed different" — they were "newer in color" and had a different
general appearance from those panels lining the trailer’s side walls.
Id. at 10. Rico was placed inside the trailer, and he immediately
"alerted on the front wall." Id. The plywood panels were then
removed and several black duffle bags and a Heineken beer box were
discovered. The duffle bags contained approximately 500 pounds of
marijuana, and the Heineken beer box contained more than $300,000
in United States currency. Id. The officers promptly seized the mari-
juana and currency and took Singh and Khangura into custody.

                                     B.

   Less than a month later, on March 31, 2003, Singh and Khangura
were indicted by the grand jury in Greensboro. Singh filed a suppres-
sion motion on April 16, 2003, asserting that the search of the trailer
was unconstitutional because it was unsupported by either a warrant
or consent. The motion was adopted by Khangura the following day.
Singh later filed a supplemental motion to suppress, asserting that the
seizure resulted from an unconstitutional Terry stop. See Terry v.
Ohio, 392 U.S. 1 (1968) (holding that investigatory stops must be
supported by reasonable suspicion of criminal activity).6 Williamson,
Huff, and various law enforcement officers testified at the evidentiary
hearing on these motions.

   After assessing the evidence under Terry and its progeny, the dis-
trict court filed its Opinion suppressing the marijuana and currency
seized from the Rig’s trailer. In so doing, the court focused on
whether the detention of Singh and Khangura by the officers "‘was
    5
    The drug detection dog was trained to alert to marijuana, cocaine,
crack cocaine, heroin, or methamphetamine by barking, biting, or
scratching in the area of a detected substance. J.A. 273.
  6
    An investigatory Terry stop implicates Fourth Amendment jurispru-
dence. The Fourth Amendment guarantees, in pertinent part, "[t]he right
of the people to be secure in their persons, houses, papers, and effects,
against unreasonable searches and seizures." U.S. Const. amend. IV.
                         UNITED STATES v. SINGH                          7
reasonably related in scope to the circumstances which justified the
interference in the first place.’"7 Opinion at 23 (quoting Terry, 392
U.S. at 20). The court then analyzed the evidence against the "proper
investigative scope for a routine traffic stop," relying on our decision
in United States v. Rusher, 966 F.2d 868, 876 (4th Cir. 1992) (dis-
cussing proper scope of routine traffic stop). Opinion at 13.

   The court concluded that the officers had exhibited such a show of
authority that "a reasonable person in the position of either of the
Defendants stopped alongside I-85 would not have felt free to decline
the officers’ requests or otherwise terminate the encounter." Id. at 14.
As a result, the court found that Singh and Khangura "were seized at,
if not before, the point when . . . Singh was summoned from Mr.
Huff’s wrecker and escorted to the front of Mr. Williamson’s wrecker
where both Defendants were then questioned." Id. at 15. Having
decided that Singh and Khangura had been "seized," the court turned
to whether the officers possessed a reasonable suspicion of criminal
activity sufficient to warrant their seizure. In addressing this issue, the
court found that "this was an ordinary traffic stop," and that a reason-
able suspicion did not develop by the time the warning citation had
been issued. Id. at 19, 23. In so ruling, the court observed that "the
Government has failed to meet its ‘burden to articulate facts sufficient
to support reasonable suspicion,’ given the totality of the circum-
stances." Id. at 23 (quoting United States v. Burton, 228 F.3d 524, 528
(4th Cir. 2000)).

   Finally, the court turned to the information obtained from the CI
in Atlanta. It observed that the officers failed "to verify any of this
information during their stop" in order to use it for a "reasonable sus-
picion that justified the seizure and questioning of the Defendants."
Id. at 24. Indeed, according to the court, the only confirmation of the
Tip was "that they located a tractor trailer that had been broken down
at milepost 202 on I-85." Id. The court — relying on testimony that
the officers stopped Williamson’s tow truck for speeding and that the
Tip was irrelevant to the stop — disregarded the Tip and its corrobo-
  7
   The propriety of an investigative detention under Terry turns on two
inquiries — whether an officer’s action was justified at its inception, and
whether the scope of the detention was reasonably related to the circum-
stances justifying the interference. Terry, 392 U.S. at 20.
8                         UNITED STATES v. SINGH
rating evidence in assessing whether the seizure of Singh and
Khangura was supported by a reasonable, articulable suspicion that
criminal activity was afoot.8 Id. at 25. The court then decided that the
search of the trailer was unconstitutional and held that the evidentiary
use of the marijuana and currency must be suppressed. Id. at 25-26.
The Government has appealed the suppression of its evidence, and we
possess jurisdiction pursuant to 18 U.S.C. § 3731.

                                     II.

   In assessing issues of reasonable suspicion and probable cause in
a search and seizure context, we review de novo the legal determina-
tions of a district court. Park v. Shiflett, 250 F.3d 843, 849 (4th Cir.
2001) (observing that determinations of reasonable suspicion and
probable cause are reviewed de novo) (citing Ornelas v. United
States, 517 U.S. 690, 699 (1996)). On the other hand, we review a
district court’s factual findings on issues of reasonable suspicion and
probable cause for clear error, giving due weight to inferences justifi-
ably drawn from those facts. Ornelas, 517 U.S. at 699. A factual find-
ing is clearly erroneous when, through our review of the evidence, we
are left with the definite and firm conviction that a mistake has been
committed. United States v. U.S. Gypsum Co., 333 U.S. 364, 395
(1948).

  A determination of reasonable suspicion or probable cause does not
depend on any single factor, but on the totality of the circumstances.
United States v. Sokolow, 490 U.S. 1, 8 (1989) (reasonable suspicion);
    8
   Sergeant Grey confirmed that the officers were "looking for the truck"
and that the purpose of multiple officers travelling "four to five counties
was to find this broken-down tractor-trailer and truck." J.A. 240. Grey
also agreed that he would not "have been there to stop the wrecker if not
for the [T]ip," as the Tip provided the "motive" to find the Rig. J.A. 249.
Officer Siler confirmed this point, agreeing that the "investigation didn’t
center around a speeding truck. It centered around [the] [T]ip." J.A. 105.
Trooper Strader further confirmed that the officers "spent three hours
running around the highways of . . . North Carolina chasing down a [bro-
ken down] tractor-trailer." J.A. 166. According to agent Bryant, reliance
on a traffic violation to justify the stop was an effort to "wall off" the CI,
apparently to protect the CI’s identity. J.A. 82.
                        UNITED STATES v. SINGH                          9
Illinois v. Gates, 462 U.S. 213, 232 (1983) (probable cause). In
addressing such issues, we assess the relevant facts known to the
authorities and decide whether those facts, "from the standpoint of an
objectively reasonable police officer," give rise to reasonable suspi-
cion or probable cause. Ornelas, 517 U.S. at 696.

                                   III.

   On appeal, the Government asserts that the district court erred in
disregarding the Tip and its corroborating evidence. It further con-
tends that the court erred in characterizing the stop and seizure of the
Rig and its occupants as an "ordinary traffic stop." As explained
below, the court did err in disregarding the Tip and its corroborating
evidence. And the court’s finding that the authorities were involved
in an ordinary traffic stop was clearly erroneous. As our analysis
reflects, the investigatory seizure of the Rig and its occupants was
supported by a reasonable suspicion of criminal activity, and the
search of the trailer was premised on probable cause.

                                   A.

    First, the Government contends that the district court erred in disre-
garding the Tip and its corroborating evidence in assessing the legal-
ity of the stop and seizure of the Rig and its occupants. The
Government maintains that, when the stop occurred, the authorities
possessed a reasonable and articulable suspicion that Singh and
Khangura were engaged in criminal activity, and such suspicion justi-
fied the stop of the Rig and the seizure of its occupants. On this point,
it is elementary that the authorities are entitled to stop a moving vehi-
cle reasonably suspected of involvement in smuggling contraband,
and they may briefly detain and investigate such a vehicle and its
occupants. See, e.g., United States v. Sharpe, 470 U.S. 675, 687-88
(1985) (holding stop of vehicle and subsequent twenty-minute investi-
gatory detention reasonable where authorities observed circumstances
indicative of drug trafficking). The first issue we must assess is
whether, when this stop occurred, the authorities possessed a reason-
able suspicion that Singh and Khangura were smuggling contraband.

   A reasonable suspicion exists when law enforcement officers pos-
sess "‘a particularized and objective basis’ for suspecting the person
10                      UNITED STATES v. SINGH
stopped of criminal activity." Ornelas, 517 U.S. at 696 (quoting
United States v. Cortez, 449 U.S. 411, 417-18 (1981)). Although an
unverified tip from a known informant may alone justify a reasonable
suspicion of criminal activity, the Tip was not unverified. Indeed, it
was substantially corroborated by the developing circumstances. See
United States v. Spotts, 275 F.3d 714, 720 (8th Cir. 2002) ("A tip
from a known informant can suffice by itself to establish reasonable
suspicion for a vehicle stop, even if the police do not corroborate the
tip prior to the stop with their own independent observation.") (citing
Adams v. Williams, 407 U.S. 143, 146-47 (1972)).9

   It is uncontested that, when the stop occurred, the authorities pos-
sessed information from the CI in Atlanta that two Indian males were
transporting marijuana from Canada to Atlanta; that they were driving
a silver tractor-trailer bearing Canadian plates; and that the Rig had
broken down in North Carolina near milepost 202 on I-85. In addition
to being a known informant, the CI was in the presence of agent
Green as the CI was receiving information from Singh and Khangura.
The officers verified the CI’s information that a silver tractor-trailer
bearing Canadian plates had broken down near milepost 202 on I-85
in North Carolina. Furthermore, the officers generally corroborated
the Tip with respect to Singh’s and Khangura’s gender and national
origin. In sum, the Tip was provided by a known informant who was
obtaining real-time information directly from Singh and Khangura,
and the Tip proved to be reliable. See Alabama v. White, 496 U.S.
325, 330-31 (1990) (observing that, in assessing tip’s credibility,
court must weigh its reliability and basis of informant’s knowledge).

   In light of the Supreme Court’s decision in Maryland v. Macon,
472 U.S. 463 (1985), the district court erred as a matter of law when
it decided to disregard the Tip and its corroborating evidence. In
Macon, the Court held that an officer’s actions in Fourth Amendment
situations are to be viewed objectively by a reviewing court, taking
into account all relevant facts and circumstances. In rejecting a sub-
jective assessment of such an officer’s state of mind, the Court
observed that:
  9
   As the Supreme Court has observed, a tip from a known informant is
entitled to greater weight than a tip from an anonymous informant.
Adams, 407 U.S. at 147.
                         UNITED STATES v. SINGH                          11
     Whether a Fourth Amendment violation has occurred "turns
     on an objective assessment of the officer’s actions in light
     of the facts and circumstances confronting him at the time,"
     and not on the officer’s actual state of mind at the time the
     challenged action was taken.

Id. at 470-71 (quoting Scott v. United States, 436 U.S. 128, 136
(1978)) (emphases added). Pursuant to Macon, the district court was
obliged to objectively assess whether, in the totality of the circum-
stances, the officers possessed a reasonable suspicion justifying the
stop and seizure of the Rig and its occupants; the subjective views of
the officers were simply irrelevant. See United States v. Hassan El,
5 F.3d 726, 729-31 (4th Cir. 1993) (adopting "purely objective" stan-
dard for evaluating whether officer’s allegedly pretextual traffic stop
comported with Fourth Amendment); Martin v. Gentile, 849 F.2d
863, 869 (4th Cir. 1988) ("Subjectively bad intentions on the part of
the individual officer will not make a constitutional violation out of
an otherwise reasonable seizure . . . .") (citations omitted).

   In the totality of the circumstances, including the Tip and its cor-
roborating evidence, the officers possessed a reasonable suspicion,
when the Rig was stopped, that it contained contraband and that its
occupants were smuggling that contraband from Canada to Georgia.
Strikingly, the district court appeared to acknowledge that the Tip,
had it been considered, would alone have justified stopping the Rig,
seizing its occupants, and searching the trailer. During the suppres-
sion hearing, the court observed that if the Tip had been part of the
totality of the circumstances, the seizure would have been justified
under Terry. The court stated that "[t]he reliability of the tip, whatever
they say, whether it’s [Interstate] 85, [Interstate] 40, there’s enough
there for that in and of itself to be sufficient for the officers to do what
they did." J.A. 379. In ruling the Tip to be irrelevant, however, the
court ignored the principles of Macon and its progeny, and it failed
to objectively assess the totality of the relevant circumstances. See,
e.g., Hassan El, 5 F.3d at 730-31; Martin, 849 F.2d at 869.

                                    B.

   Because the Tip and its corroborating evidence were disregarded,
the court erroneously found, as a factual matter, that the officers were
12                      UNITED STATES v. SINGH
involved in an "ordinary traffic stop." And the court’s determination
that this was an ordinary traffic stop led it to analyze the situation
under our decision in Rusher. Opinion at 19. We are, however, pre-
sented with markedly different circumstances. In Rusher, we exam-
ined a situation where moving violations precipitated a "routine traffic
stop," and where a subsequent consensual investigation uncovered
guns and narcotics. Rusher, 966 F.2d 868. We explained that the
proper investigative scope of a routine traffic stop includes requesting
a driver’s license and vehicle registration, running a computer check,
and issuing a citation. Id. at 876. If the driver produces a valid license
and proof that he is entitled to operate the vehicle, he may, upon issu-
ance of the citation, "proceed on his way, without being subject to
further delay by police for additional questioning." Id. Significantly,
however, nothing had occurred before the traffic stop in Rusher to
provide any suspicion that the vehicle was transporting contraband.

    Here, as Sergeant Grey confirmed, the Tip provided the basis for
the authorities’ concerted effort to locate the disabled Rig. And con-
trary to the view of the district court, the investigation of the Rig did
not center on a speeding violation — it centered on the Tip. See supra
note 8. The officers initially learned about the disabled Rig from the
informant in Atlanta. When Grey, Siler, and Strader received the Tip,
they were more than seventy miles from the disabled Rig. Upon
receiving clarifying information from the CI, an officer was dis-
patched to milepost 202 to corroborate the Tip. After the disabled Rig
was found at milepost 202, Grey, Siler, and Strader drove more than
seventy miles north on I-85 to intercept it. While proceeding north on
I-85, Grey contacted the NCHP K-9 narcotic detection unit, directed
it to milepost 202, and requested that it stand by to sweep the Rig. In
the totality of the circumstances, we are led to the definite and firm
conviction that a mistake has been committed, and the court’s finding
that this was an "ordinary traffic stop" is clearly erroneous. As we
have explained, the record establishes that the investigation of the Rig
and its occupants centered on the Tip — not on a speeding tow truck.

                                   C.

   Finally, we turn to whether the search of the trailer and the seizure
of the marijuana and currency were constitutionally permissible. As
the Supreme Court has explained, "[t]he scope of [a] search must be
                        UNITED STATES v. SINGH                        13
‘strictly tied to and justified by’ the circumstances which rendered its
initiation permissible." Terry, 392 U.S. at 19 (quoting Warden v. Hay-
den, 387 U.S. 294, 310 (1967) (Fortas, J., concurring)). Where rea-
sonable suspicion exists to support the stop of a moving vehicle,
officers are entitled to conduct an investigatory detention to obtain
consent to search or to develop probable cause. See United States v.
Brignoni-Ponce, 422 U.S. 873 (1975) (recognizing, in context of
investigatory stop, that authorities may question driver and passengers
about suspicious circumstances); see also Sharpe, 470 U.S. 675 (hold-
ing twenty-minute investigatory detention reasonable following vehi-
cle stop). In this situation, the detention of Singh and Khangura was
both brief and non-intrusive and, when the search occurred, there was
probable cause to believe that marijuana would be discovered in the
trailer.

   During the stop, the officers obtained significant additional infor-
mation bearing on the probable cause issue. This information included
Singh being "visibly nervous and agitated" when Strader stopped Wil-
liamson’s tow truck; Singh hurriedly attempting to place a call on his
cell phone; Singh and Khangura requesting that the tractor be repaired
immediately "no matter what the price"; Singh and Khangura request-
ing that the trailer be towed to a different location so that it could be
"picked up"; Singh and Khangura being "nervous"; and the tow truck
drivers viewing the behavior of Singh and Khangura to be "strange"
and "unusual." See supra p. 4. This additional information, along with
the Tip and its corroborating evidence, rendered the questioning of
Singh and Khangura reasonable and permissible.

   The responses made by Singh and Khangura after the stop occurred
warranted a heightened suspicion that criminal activity was afoot.
These responses revealed that Singh and Khangura were not experi-
enced truckers, and that they claimed to be travelling unloaded from
Boston, hoping to secure a load in Charlotte. They also provided the
officers with conflicting information regarding Yadu Transport. See
supra p. 5. Given the views of the tow truck drivers that Singh and
Khangura were nervous, anxious, and agitated, and the officers’ views
that Singh’s and Khangura’s responses were suspicious, further inves-
tigation was warranted.
14                       UNITED STATES v. SINGH
   Although the Tip, its corroborating evidence, and the roadside
investigation may have warranted a search of the trailer, the officers
took further precautions. First, they secured Singh’s and Khangura’s
written consent to search.10 Second, they utilized their narcotic detec-
tion dog Rico to examine the outside of the trailer. The dog promptly
alerted to the odor of narcotics at the left side of the front of the trailer
and at the trailer’s rear doors. Those reactions, coupled with the other
relevant circumstances, provided probable cause for the actions of the
officers. See Florida v. Royer, 460 U.S. 491, 505-06 (1983) (recog-
nizing that drug dog’s positive alert establishes probable cause);
United States v. McFarley, 991 F.2d 1188, 1193 (4th Cir. 1994)
(same). In sum, an assessment of the totality of the circumstances
reveals that the officers possessed probable cause to search the trailer.11

                                    IV.

   Pursuant to the foregoing, we reverse the suppression of the mari-
juana and currency seized from the trailer, and we remand for addi-
tional proceedings consistent with this opinion.

                                          REVERSED AND REMANDED

  10
    The district court did not reach the consent issue, and we also need
not address that issue. The efforts to obtain consent, however, reflect on
the reasonableness of the officers’ conduct. In any event, consent was
unnecessary if the search of the trailer was otherwise permissible.
  11
    A warrantless search of the trailer was justified under the vehicular
exception to the Fourth Amendment’s warrant requirement. United
States v. Gastiaburo, 16 F.3d 582 (4th Cir. 1994) (observing that war-
rantless search of immobilized vehicle is constitutional where authorities
had probable cause to believe that contraband would be found); see also
California v. Carney, 471 U.S. 386, 391 (1985) ("Even in cases where
an automobile [is] not immediately mobile, the lesser expectation of pri-
vacy resulting from its use as a readily mobile vehicle justifie[s] applica-
tion of the vehicular exception.").
