If, on such examination, it is made to appear that the person so arrested is a citizen of the United States, he shall be forthwith discharged from arrest, and shall be left to the ordinary course of law. But if this is not made to appear, and such court, judge, or magistrate judge finds, upon the papers referred to in section 257 of this title, a sufficient prima facie case that the matter concerns only the internal order and discipline of such foreign vessel, or whether in its nature civil or criminal, does not affect directly the execution of the laws of the United States, or the rights and duties of any citizen of the United States, he shall forthwith, by his warrant, commit such person to prison, where prisoners under sentence of a court of the United States may be lawfully committed, or, in his discretion, to the master or chief officer of such foreign vessel, to be subject to the lawful orders, control, and discipline of such master or chief officer, and to the jurisdiction of the consular or commercial authority of the nation to which such vessel belongs, to the exclusion of any authority or jurisdiction in the premises of the United States or any State thereof. No person shall be detained more than two months after his arrest, but at the end of that time shall be set at liberty and shall not again be arrested for the same cause. The expenses of the arrest and the detention of the person so arrested shall be paid by the consular officers making the application: Provided, That nothing in this section or section 257 of this title shall authorize the arrest or imprisonment of officers and seamen deserting or charged with desertion from merchant vessels of foreign nations in the United States and Territories and possessions thereof, and the cooperation, aid, and protection of competent legal authorities in effecting such arrest or imprisonment.