There's no dispute that the traffic stop one morning in May 2009 was legitimate. The driver of a Pontiac Grand Prix failed to use a signal before turning into an exit lane to leave Interstate 95 in South Carolina.
The authorities initiated a traffic stop on the exit ramp that morning near Charleston. The passenger, police would later say, was nervous. His arms and legs were shaking, according to court records.
The stop wasn't random. Local and federal investigators, working on a drug case, secretly were monitoring the movement of the car via a global positioning device—one that had been attached without a warrant.