15 U.S.C. § 23

In any suit, action, or proceeding brought by or on behalf of the United States subpoenas for witnesses who are required to attend a court of the United States in any judicial district in any case, civil or criminal, arising under the antitrust laws may run into any other district: Provided , That in civil cases no writ of subpoena shall issue for witnesses living out of the district in which the court is held at a greater distance than one hundred miles from the place of holding the same without the permission of the trial court being first had upon proper application and cause shown.