28 C.F.R. § 21.7
In any case in which the U.S. Department of Justice, or office or organization thereof, is a party, the Department of Justice shall pay all fees and allowances of witnesses, except for those witnesses as defined in §21.2, paragraph (d)(1), on the certification of the following officials: The U.S. Attorney, an Assistant U.S. Attorney, a U.S. Trustee, or the U.S. Department of Justice attorney who actually conducts the case. In criminal proceedings in forma pauperis or in proceedings before a U.S. Commissioner, U.S. Magistrate or U.S. Parole Commission Hearing Examiner, the Department of Justice shall pay all fees and allowances of witnesses on the certification of the U.S. District Judge hearing the case or such Commissioner, Magistrate, or Hearing Examiner.
28 C.F.R. § 21.7