Appellant was stopped for a speeding violation by an Illinois state trooper. A second trooper who overheard the transmission drove to the scene with his narcotics-detection dog and walked the dog around the car while the first trooper wrote a ticket. When the dog alerted to the trunk, the officers searched it, finding marijuana. The trial court denied his motion to suppress, and he was convicted. The Illinois Supreme Court reversed, finding that because there were no specific and articulable facts to suggest drug activity, use of the dog unjustifiably expanded a traffic stop into a drug investigation. The United States Supreme Court granted certiorari on the question of whether the Fourth Amendment requires reasonable articulable suspicion to justify using a drug-detection dog to sniff a vehicle during a traffic stop. The Court held that a dog sniff during a lawful traffic stop which reveals no information other than the location of an unlawfully possessed substance does not violate the Fourth Amendment. J. Souter and Ginsburg dissented.