The doctrine derives from the 1925 case of Carroll v. the United States, in which bootleg whiskey being smuggled into Michigan from Canada was seized in a search of the suspect's automobile under circumstances unrelated to a search incident to lawful arrest. The exigency for this type of search arises from the mobility of the automobile, which sets cars or any other mobile vehicles apart from the constitutionally protected, immobile threshold of the home. Probable cause is the second requirement needed to legitimize a warrantless search under the Carroll Doctrine. Because the scope of search incident law was broad, the Carroll Doctrine remained undeveloped in ensuing decades. Since the case of Chimel in 1969, in a period of restricted interpretation of search incident, other cases have contributed to the fleshing out of the Carroll Doctrine, defining the perimeters of warrantless search in terms of sections of the vehicle or police relocation of the vehicle.