D.C. Code § 34-919
Each witness who shall appear before the Commission or its agent by its order shall receive for his attendance the fees and mileage provided for witnesses in the United States District Court for the District of Columbia on March 4, 1913, which shall be audited and paid in the same manner as fees in criminal cases within the District of Columbia are audited and paid, upon the presentation of proper vouchers, sworn to by such witnesses and approved by the chairman of the Commission. No witnesses subpoenaed at the instance of parties other than the Commission shall be entitled to compensation for attendance or travel unless the Commission shall certify that his testimony was material to the matter investigated, and that his attendance as a witness was reasonably necessary.
D.C. Code § 34-919