8 CFR § 1235.5 - Preinspection.
(a) In United States territories and possessions. For provisions of the DHS regulations with respect to examinations of passengers and crew in the case of any aircraft proceeding from Guam, the Commonwealth of the Northern Mariana Islands (beginning November 28, 2009), Puerto Rico, or the United States Virgin Islands destined directly and without touching at a foreign port or place, to any other of such places, or to one of the States of the United States or the District of Columbia, see 8 CFR 235.5.
(b) In foreign territory. In the case of any aircraft, vessel, or train proceeding directly, without stopping, from a port or place in foreign territory to a port-of-entry in the United States, the examination and inspection of passengers and crew required by the Act and final determination of admissibility may be made immediately prior to such departure at the port or place in the foreign territory and shall have the same effect under the Act as though made at the destined port-of-entry in the United States.