28 C.F.R. § 0.111a
The Director of the United States Marshals Service and officers of the United States Marshals Service designated by him are authorized to exercise the power and authority vested in the Attorney General under 18 U.S.C. 3508 to receive custody from foreign authorities of prisoner-witnesses whose temporary transfer to the United States has been requested; to transport such persons in custody from the cooperating foreign country to the place in the United States at which the criminal proceedings in which they are to testify are pending; to maintain such persons in custody while they are in the United States, subject to any agreement entered into by the Assistant Attorney General for the Criminal Division or his or her delegee with the transferring country regarding the terms or conditions of the transfer; and to return such persons, in custody, to the foreign country when and in the manner designated by the Assistant Attorney General for the Criminal Division or his or her delegee. The Director of the United States Marshals Service and officers of the United States Marshals Service designated by him shall also be authorized to transport, surrender, receive and maintain custody of prisoner-witnesses temporarily transferred from or to the United States pursuant to a treaty, executive agreement, or other legal authority, and accept reimbursement from foreign authorities when appropriate.
28 C.F.R. §0.111a