Iowa State Trooper Mark Anderson stopped a vehicle proceeding north on Interstate 35 for having excessively dark windows. At the conclusion of the traffic stop, driver Kou Yang consented to a search of the vehicle. The search yielded some suspicious items but no contraband. Yang then said he wanted to leave, but Anderson detained the vehicle until a drug dog could arrive. When the dog alerted to the presence of narcotics, Anderson took the vehicle to a nearby truck stop for a more thorough search. The second search yielded no contraband, but Anderson could not find Yang to return the vehicle. The following day, with the vehicle still unclaimed, Anderson obtained a search warrant. The warrant search uncovered one and one-half pounds of methamphetamine hidden in the vehicle. Yang was indicted for possession with intent to distribute.
Yang moved to suppress the methamphetamine. After a hearing, the magistrate judge recommended suppression because Yang’s consent to search was involuntary. The district court disagreed but granted the motion to suppress on another ground — that Anderson lacked reasonable suspicion to detain the vehicle after Yang revoked his consent. The government appeals. Reviewing the district court’s factual findings for clear error and its conclusions of law de novo, we reverse. United States v. Wells, 223 F.3d 835, 838 (8th Cir.2000) (standard of review).
I. Background
The traffic stop occurred at 11:33 a.m. on July 17, 2001. While asking Yang for his driver’s license, vehicle registration, and proof of insurance, Trooper Anderson saw numerous food items, a cell phone, and an atlas on the passenger seat, and a roll of toilet paper on the floor. Yang volunteered that he had flown to Dallas, Texas, to purchase the vehicle. The certificate of title showed a transfer of title to Yang and a handwritten odometer reading of 187,000 miles. Yang volunteered that he had made some repairs to the vehicle in Texas.
While Anderson completed the traffic stop paperwork, he questioned Yang about his travel to purchase the car. Yang said that after his wife saw the car on the internet, he paid $5,000 for the car, sending a money order for half the purchase price before flying to Dallas. Yang said he had flown to Dallas from Minneapolis on July 11th and left Dallas on the 12th. Anderson commented that five days seemed too long to drive from Dallas to Iowa. When Yang said he was a co-owner of a temporary help agency in Minneapolis, Anderson asked if Yang was in a hurry to get home. Yang said he was but would only drive “200 miles or so [before] I’d *653stop and rest so I wouldn’t get in a car accident.”
Anderson issued a warning citation at 11:47 a.m. and told Yang he was free to go. As Yang left the patrol car, Anderson asked, “Would there be any reason why somebody would ah say that you are hauling narcotics today?” Yang calmly said no, denied having any drugs with him, and walked toward his car. As he reached the driver’s side door, the following exchange took place:
Anderson: Sir! You’re free to go and all but I, would it be alright if I searched your car?
Yang: Yea.
Anderson: ... I just have a piece of paper I’d like you to sign if you would for me. If that’s okay. I’ll show it here to ya. It’s a consent to search form .... giving me permission to search your vehicle if that’s alright.
Yang: But I could just leave? Or what?
Anderson: Yes, you can.
Yang: Oh, then I’m just going to go.
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Anderson: You don’t want to sign this?
Yang: I do not.
* * * ‡ *
Anderson: Like I said, this is just a consent to search form. It’s like what says that I could do it, it’s just for my office, for my boss. Um, if that’s all right, I’ll just have you sign by the star, but you don’t have to either if you don’t want to.
Yang: I don’t, but I can open the trunk (unintelligible) everything and you can see it.
Anderson: You just don’t want to sign?
Yang: No.
Anderson: But it’s all right if I search your vehicle though?
Yang: Yeah.
Yang opened the trunk at 11:49 a.m. At about this time, Iowa State Trooper David Baker arrived. When Anderson finished searching the trunk, he turned to Yang and said, gesturing towards the car’s passenger compartment, “All right if I look? That okay?” Yang nodded affirmatively. Anderson began searching the car at 11:52 a.m.
In the car, Anderson found a July 11 receipt from a tire store in El Paso, Texas and a July 12 receipt from an auto glass shop in San Diego, California. Anderson asked Yang if he had been in California; Yang responded he had visited his mother in San Diego. Anderson also found a set of new screwdrivers; Yang explained he bought the screwdrivers to open the car door after he locked his key in the car. Anderson testified that Yang began to appear nervous at this point. At 12:03 p.m., Anderson and Baker summoned a K-9 unit from Iowa Falls. At 12:14, Anderson told Yang his story did not make sense and said that a drug dog would soon arrive. Yang said they had searched long enough and he wanted to go. Anderson said Yang could leave but they would detain the vehicle for a dog sniff. Yang declined a ride to a nearby truck stop and waited with the vehicle. At 12:31 p.m., an Iowa Falls K-9 unit arrived. When the dog alerted on the car a few minutes later, the troopers told Yang they would search the car further and then release it to Yang if nothing was found. At about 1:00 p.m., Yang agreed to drive the car to a safer location, a truck stop seven miles north of the traffic stop.
When they arrived, Yang got out of his car and walked towards the truck stop. The officers resumed their search of the car. They again found no contraband. *654Anderson went to the truck stop to tell Yang he could leave with the car. Truck stop employees said that Yang had left the truck stop and not returned. Anderson filled out a tow sheet and inventory form and placed a “no hold” on the vehicle, which meant that Yang could reclaim it. After telling the truck stop employees that Yang was free to take the vehicle, Anderson and Baker left the truck stop that evening. The truck stop owner stored Yang’s car in a locked metal shed. The following day, Anderson obtained a warrant to search the vehicle and returned to the truck stop to conduct the warrant search. Using the screwdrivers found in the car, Anderson removed a speaker cover and some molding and found the methamphetamine hidden inside the passenger door.
II. The Magistrate Judge’s Decision
After an evidentiary hearing on Yang’s motion to suppress, the magistrate judge recommended the motion be granted because (i) at the end of the traffic stop, Trooper Anderson had no “concrete reasons” to suspect Yang of criminal activity and therefore no reasonable suspicion justifying continued detention; (ii) Anderson then illegally detained Yang by asking him for permission to search the car; and (iii) Yang’s consent to search did not purge the taint of this unlawful detention because the consent was coerced when Anderson “lied to Yang about the consent form” and twice prevented Yang from leaving by drawing him into further conversation after saying he was free to go. The magistrate judge also observed that, if the consent to the initial search was in fact voluntary, the discovery of “suspicious paperwork from California,” after which “Yang changed his story about his itinerary,” gave Anderson “ample reason to detain the vehicle until the drug dog arrived.” The district court rejected the magistrate judge’s analysis, concluding that Yang’s consent to the initial search was voluntary and therefore purged the taint of any prior illegal detention. We agree with the district court.
In the first place, the magistrate judge erred in concluding that Trooper Anderson unlawfully detained Yang by asking him for permission to search the car after the traffic stop was over. Law enforcement officers do not violate the Fourth Amendment by asking a person for consent to search or other types of cooperation, even when they have no reason to suspect that person, “provided they do not induce cooperation by coercive means.” United States v. Drayton, 536 U.S. 194, 201, 122 S.Ct. 2105, 153 L.Ed.2d 242 (2002); see United States v. Jones, 269 F.3d 919, 925 (8th Cir.2001). That being so, the time it takes for an officer to find out if consent will be given cannot be an unlawful detention in the absence of coercive or otherwise unusual circumstances. Here, after completing the traffic stop, Anderson told Yang he was free to go and asked for consent to search the car. Yang immediately said, “Yea,” and after a few additional moments of conversation opened the trunk of the car for Anderson. There was no unlawful detention.
Second, even if Anderson’s request for consent amounted to illegal detention, we agree with the district court that the illegality was purged by Yang’s voluntary consent to the search. See United States v. Kreisel, 210 F.3d 868, 869 (8th Cir.), cert. denied, 531 U.S. 916, 121 S.Ct. 273, 148 L.Ed.2d 198 (2000) (consent purges illegal taint). In response to Anderson’s initial request, Yang voluntarily consented to the search. Anderson then used a bit of deception in trying to persuade Yang to sign a consent-to-seareh form. We do not approve of that tactic, but it did not have a constitutionally coer*655cive effect. Yang persisted in refusing to sign the form but confirmed his oral consent to a search by opening the trunk for Anderson. After the trunk was searched, Yang signaled with a nod that Anderson could search the vehicle’s passenger compartment. When oral consent is voluntarily given, the absence of a signed consent form is immaterial. United States v. Martel-Martines, 988 F.2d 855, 859 (8th Cir.1993). The district court did not err in finding that the eonsent-to-search was voluntary. See United States v. Moreno, 280 F.3d 898, 900-01 (8th Cir.2002).
III. The District Court’s Decision
The district court nonetheless granted Yang’s motion to suppress, concluding that Troopers Anderson and Baker violated the Fourth Amendment at a later point in the encounter. Forty minutes after the traffic stop, when the troopers advised Yang they had called for a drug dog, he said they had searched long enough and he wanted to leave. The government does not challenge the court’s finding that this was a revocation of the prior consent-to-search. Thus, the issue is whether, at that moment, the troopers had reasonable suspicion that other criminal activity may be afoot, justifying their decision to detain the vehicle until the drug dog arrived some fifteen minutes later. The government argues that many circumstances provided reasonable suspicion that Yang’s vehicle was transporting illegal drugs. The district court individually analyzed those circumstances and concluded they “appear to be nothing more than a collection of innocent circumstances” that did not provide reasonable suspicion to further detain Yang’s vehicle. Like the magistrate judge, we disagree.
The Supreme Court has said repeatedly that courts must look at the totality of the circumstances when deciding whether there was reasonable suspicion supporting an investigatory detention. United States v. Arvizu, 534 U.S. 266, 273, 122 S.Ct. 744, 151 L.Ed.2d 740 (2002). Thus, even if Yang’s individual actions might be innocently explained, his behavior “must be considered as a whole and in the light of the officers’ ‘experience and specialized training.’ ” United States v. Ameling, 328 F.3d 443, 448 (8th Cir.2003) (quoting Arvizu, 534 U.S. at 273, 122 S.Ct. 744). In Arvizu,, the Court reversed a suppression order, criticizing the court of appeals for evaluating seven factors “in isolation from each other” and giving them “no weight” because each “was by itself readily susceptible to an innocent explanation.” 534 U.S. at 274, 122 S.Ct. 744. Similarly, the district court erred by analyzing the circumstances in isolation, concluding that flying from Minneapolis to Texas to purchase a high-mileage car created “no specific, articulable basis” to suspect drug trafficking; failure to disclose the trip to California “could not generate a reasonable suspicion”; the five days it apparently took to drive from Dallas to Iowa was innocently explained by Yang’s later admission he had also traveled to California; no “reasonable suspicion of criminality” may be gleaned from the fact Yang was traveling from a drug source area; and finding a cell phone and screwdrivers in the car and observing half-eaten food and toilet paper on the floor of the car were “entirely consistent with innocent travel.”
An officer making a traffic stop may ask the driver his destination and the purpose for his trip. United States v. Linkous, 285 F.3d 716, 719 (8th Cir.2002). The answers to these routine questions may arouse suspicion that other criminal activity is afoot, in which case the officer may ask additional questions to verify or dispel his suspicion. United States v. Ra*656mos, 42 F.3d 1160, 1163 (8th Cir.1994), cert. denied, 514 U.S. 1134, 115 S.Ct. 2015, 131 L.Ed.2d 1013 (1995). The officer may ask for consent to search and if consent is given, act on whatever information he acquires. United States v. Martinez, 168 F.3d 1043, 1047 (8th Cir.1999). “An officer’s suspicion of criminal activity may reasonably grow over the course of a traffic stop as the circumstances unfold and more suspicious facts are uncovered.” Linkous, 285 F.3d at 720.
In this case, Anderson testified that his suspicion was initially aroused because Yang was coming from a drug source state; had flown to Texas to purchase a high-mileage car and to drive it back to Minneapolis; said he was in a hurry to get home but was driving well under the speed limit and had taken five or six days to travel from Dallas to Iowa; was traveling with a cell phone and with items indicating he did not want to stop; and gave uncommon answers to questions about crime and narcotics. During the initial consensual search, Anderson’s suspicion was heightened when he found a new set of screwdrivers and receipts revealing tire repair in El Paso and glass repair in San Diego, California. When questioned about these items and his trip to California, Yang for the first time became agitated and “he started to pace back and forth,” paying much closer attention to Anderson and the car. Shortly thereafter, Anderson told Yang that a drug dog had been summoned. Yang immediately said they had searched long enough and he wanted to leave. See United States v. Green, 52 F.3d 194, 199-200 (8th Cir.1995) (revocation of consent may contribute to reasonable suspicion). In these circumstances, we conclude the troopers had reasonable suspicion that the car might be transporting illegal drugs, which warranted a brief detention until the drug dog arrived and sniffed the vehicle. See United States v. Bloomfield, 40 F.3d 910, 917 (8th Cir.1994) (en banc) (one-hour detention was lawful), cert. denied, 514 U.S. 1013, 115 S.Ct. 1353, 131 L.Ed.2d 212 (1995); United States v. White, 42 F.3d 457, 460 (8th Cir.1994) (eighty-minute wait for a drug dog was reasonable).
When the dog alerted on Yang’s vehicle, the troopers had probable cause to search the vehicle without a warrant. United States v. $404,905.00 in U.S. Currency, 182 F.3d 643, 647 (8th Cir.1999), cert. denied, 528 U.S. 1161, 120 S.Ct. 1175, 145 L.Ed.2d 1083 (2000). Accordingly, there was no Fourth Amendment violation prior to Trooper Anderson obtaining the warrant, and we need not consider the government’s alternative argument that Yang abandoned the vehicle at the truck stop before the warrant was obtained.
The district court’s order granting Kou Yang’s motion to suppress is reversed.