After an Illinois state trooper stopped respondent for speeding and radioed in, a second trooper, overhearing the transmission, drove to the scene with his narcotics-detection dog and walked the dog around respondent'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 unjustifiably 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 Amendment. Pp. 2-4.

Full Text of Opinion