The dog detected a shipment of marijuana in the trunk. Caballes was convicted of drug trafficking and sentenced to 12 years in prison and fined more than a quarter-million dollars.
In 2003, the Illinois Supreme Court overturned his conviction, ruling that the marijuana evidence should have been excluded from his trial because the dog search was illegal.
But, as Justice John Paul Stevens wrote for a six-member majority of the Supreme Court yesterday, 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."
Racial profiling was a background issue in the case. Caballes was supported by the National Association of Criminal Defense Lawyers, which told the justices in a friend-of-the-court brief that "controlling the discretion of the police may well be the most effective way to address this [profiling] problem."
Caballes had been pulled over for driving six miles per hour over the posted 65-mph speed limit. The officer who stopped him testified that his suspicions increased when he saw that Caballes seemed nervous and said he was moving to Chicago but had no luggage other than a pair of sport coats.
But the majority did not directly address the validity of the officer's deductions or racial profiling generally. Stevens noted that the state courts had already found 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."
Stevens was joined by Justices Sandra Day O'Connor, Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Stephen G. Breyer. Justices David H. Souter and Ruth Bader Ginsburg dissented. Chief Justice William H. Rehnquist, ill with cancer, did not vote.
"A drug-detection dog is an intimidating animal," Ginsburg wrote. "Under today's decision, every traffic stop could become an occasion to call in the dogs, to the distress and embarrassment of the law-abiding population."
But both Ginsburg and Souter said they might not have objected to a warrantless dog sniff to discover explosives or other possible terrorist weapons.
The Bush administration had urged the justices to uphold the dog sniff because such searches are a major part of the federal government's homeland security efforts.
The case is Illinois v. Caballes, No. 03-923.