19 U.S.C. § 1595: Searches and seizures

any merchandise upon which the duties have not been paid, or which has been otherwise brought into the United States unlawfully; any property which is subject to forfeiture under any provision of law enforced or administered by the United States Customs Service; or any document, container, wrapping, or other article which is evidence of a violation of section 1592 of this title involving fraud or of any other law enforced or administered by the United States Customs Service, If any house, store, or other building or place, in which any merchandise or other article subject to forfeiture is found, is upon or within 10 feet of the boundary line between the United States and a foreign country, such portion thereof that is within the United States may be taken down or removed. Any person authorized by this chapter to make searches and seizures, or any person assisting him or acting under his directions, may, if deemed necessary by him or them, enter into or upon or pass through the lands, inclosures, and buildings, other than the dwelling house, of any person whomsoever, in the discharge of his official duties.