18 U.S.C. § 21

(a) Wherever in this title it is an element of an offense that— (1) any property was embezzled, robbed, stolen, converted, taken, altered, counterfeited, falsely made, forged, or obliterated; and (2) the defendant knew that the property was of such character; such element may be established by proof that the defendant, after or as a result of an official representation as to the nature of the property, believed the property to be embezzled, robbed, stolen, converted, taken, altered, counterfeited, falsely made, forged, or obliterated. (b) For purposes of this section, the term “official representation” means any representation made by a Federal law enforcement officer (as defined in section 115) or by another person at the direction or with the approval of such an officer.