27 CFR 447.56: Authority of Customs officers.

(a) Officers of the U.S. Customs Service are authorized to take appropriate action to assure compliance with this part and with 27 CFR Parts 478 and 479 as to the importation or attempted importation of articles on the U.S. Munitions Import List, whether or not authorized by permit.

(b) Upon the presentation to him of a permit or written approval authorizing importation of articles on the U.S. Munitions Import List, the Customs officer who has authority to release same may require, in addition to such documents as may be required by Customs regulations, the production of other relevant documents relating to the proposed importation, including, but not limited to, invoices, orders, packing lists, shipping documents, correspondence, and instructions.