18 U.S.C. § 755

Whoever, having in his custody any prisoner by virtue of process issued under the laws of the United States by any court, judge, or magistrate judge, voluntarily suffers such prisoner to escape, shall be fined under this title or imprisoned not more than 5 years, or both; or if he negligently suffers such person to escape, he shall be fined under this title or imprisoned not more than one year, or both.