19 CFR §145.56

Merchandise subject to regulations of the Office of Foreign Assets Control of the Treasury Department prohibiting or restricting entry of unlicensed importations of articles directly or indirectly from certain designated countries shall be detained until licensed or the question of its release, seizure, or other disposition has been determined under the Foreign Assets Control or Cuban Assets Control regulations (31 CFR parts 500 and 515) (See also 19 CFR 12.150).