39 U.S.C. § 604: Seizing and detaining letters

An officer or employee of the Postal Service performing duties related to the inspection of postal matters, a customs officer, or United States marshal or his deputy, may seize at any time, letters and bags, packets, or parcels containing letters which are being carried contrary to law on board any vessel or on any post road. The officer or employee who makes the seizure shall convey the articles seized to the nearest post office, or, by direction of the Postal Service or the Secretary of the Treasury, he may detain them until 2 months after the final determination of all suits and proceedings which may be brought within 6 months after the seizure against any person for sending or carrying the letters.