The Department of Justice shall conduct the legal proceedings necessary to enforce the provisions of sections 401, 403, 404, 406, 407, 408, 409, 411, and 412 of this title; and it shall be the duty of United States attorneys to vigorously prosecute all offenders against the same whenever requested to do so by the Secretary of the Army or by any of the officials hereinafter designated, and it shall furthermore be the duty of said United States attorneys to report to the Attorney General of the United States the action taken by him against offenders so reported, and a transcript of such reports shall be transmitted to the Secretary of the Army by the Attorney General; and for the better enforcement of the said provisions and to facilitate the detection and bringing to punishment of such offenders, the officers and agents of the United States in charge of river and harbor improvements, and the assistant engineers and inspectors employed under them by authority of the Secretary of the Army, and the United States collectors of customs and other revenue officers shall have power and authority to swear out process, and to arrest and take into custody, with or without process, any person or persons who may commit any of the acts or offenses prohibited by the said sections, or who may violate any of the provisions of the same: Provided, That no person shall be arrested without process for any offense not committed in the presence of some one of the aforesaid officials: And provided further, That whenever any arrest is made under such sections, the person so arrested shall be brought forthwith before a magistrate judge, judge, or court of the United States for examination of the offenses alleged against him; and such magistrate judge, judge, or court shall proceed in respect thereto as authorized by law in case of crimes against the United States.