Carroll v. United States was a landmark Supreme Court case that established the “automobile exception” to the Fourth Amendment’s protection against unreasonable searches and seizures. The case involved a search of a wagon by federal officers without a warrant. The officers had reasonable suspicion that the wagon contained contraband, and they found and seized a large quantity of untaxed liquor. The Supreme Court held that the search was reasonable under the Fourth Amendment. The Court reasoned that the officers had probable cause to believe that the wagon contained contraband, and that the search was necessary to prevent the contraband from being removed before a warrant could be obtained. The Court also held that the search was reasonable because the officers had no way of knowing whether the wagon was a vehicle or a stationary structure. The Court’s decision established the “automobile exception” to the Fourth Amendment, which allows law enforcement officers to search a vehicle without a warrant if they have probable cause to believe that it contains contraband. The Court’s decision has been widely cited in subsequent cases involving searches of vehicles.

In the case of Carroll v. United States, the Supreme Court was asked to decide whether a search warrant issued by an officer of the United States government could be used to search for and seize goods that were suspected of being stolen from interstate commerce. The majority opinion held that such searches were permissible under certain circumstances, but Justice Field dissented on this point. He argued that allowing federal officers to conduct such searches without probable cause violated both the Fourth Amendment's prohibition against unreasonable searches and seizures as well as Article I Section 9 which states "No bill of attainder or ex post facto law shall be passed." In his view, these laws protected citizens from arbitrary governmental interference with their property rights and should not be disregarded in favor of granting broad powers to federal officers who may have acted improperly or without sufficient evidence.