19 U.S.C. § 482

(a) Any of the officers or persons authorized to board or search vessels may stop, search, and examine, as well without as within their respective districts, any vehicle, beast, or person, on which or whom he or they shall suspect there is merchandise which is subject to duty, or shall have been introduced into the United States in any manner contrary to law, whether by the person in possession or charge, or by, in, or upon such vehicle or beast, or otherwise, and to search any trunk or envelope, wherever found, in which he may have a reasonable cause to suspect there is merchandise which was imported contrary to law; and if any such officer or other person so authorized shall find any merchandise on or about any such vehicle, beast, or person, or in any such trunk or envelope, which he shall have reasonable cause to believe is subject to duty, or to have been unlawfully introduced into the United States, whether by the person in possession or charge, or by, in, or upon such vehicle, beast, or otherwise, he shall seize and secure the same for trial. (b) Any officer or employee of the United States conducting a search of a person pursuant to subsection (a) shall not be held liable for any civil damages as a result of such search if the officer or employee performed the search in good faith and used reasonable means while effectuating such search.