Carroll v. United States, 1925. In 1921, federal prohibition officers were following George Carroll and John Cairo, who were driving from Detroit to Grand Rapids, Michigan. The officers pulled over the car and searched the vehicle. Inside the upholstery, they discovered 69 quarts of alcohol.
Because it was prohibition, Carroll and Cairo were arrested. Carroll argued in court that the evidence be suppressed because the officers did not have a warrant. The officers said that they were allowed to conduct warrantless searches of vehicles if they had a reason to believe alcohol was being transported.
This case made it all the way to the U.S. Supreme Court. In a 7-2 decision, the Supreme Court ruled that a warrantless search of an automobile was permissible under the Fourth Amendment.
The court explained that since cars were mobile, it was impracticable to obtain a search warrant if the police believe they had probable cause. This is known today as the Carroll Doctrine.