The Supreme Court ruled Tuesday that police need a warrant to search the vehicle of someone they have arrested if the person is locked up in a patrol cruiser and poses no safety threat to officers.

The court's 5-4 decision puts new limits on the ability of police to search a vehicle immediately after the arrest of a suspect.

Justice John Paul Stevens said in the majority opinion that warrantless searches still may be conducted if a car's passenger compartment is within reach of a suspect who has been removed from the vehicle or there is reason to believe evidence of a crime will be found.

"When these justifications are absent, a search of an arrestee's vehicle will be unreasonable unless police obtain a warrant," Stevens said.

Justice Samuel Alito, in dissent, complained that the decision upsets police practice that has developed since the court first authorized warrantless searches immediately following an arrest.

"There are cases in which it is unclear whether an arrestee could retrieve a weapon or evidence," Alito said.

Even more confusing, he said, is asking police to determine whether the vehicle contains evidence of a crime. "What this rule permits in a variety of situations is entirely unclear," Alito said.