5 U.S.C. § 5751: Travel expenses of witnesses

Under such regulations as the Attorney General may prescribe, an employee as defined by section 2105 of this title (except an individual whose pay is disbursed by the Secretary of the Senate or the Chief Administrative Officer of the House of Representatives) summoned, or assigned by his agency, to testify or produce official records on behalf of the United States is entitled to travel expenses under subchapter I of this chapter. If the case involves the activity in connection with which he is employed, the travel expenses are paid from the appropriation otherwise available for travel expenses of the employee under proper certification by a certifying official of the agency concerned. If the case does not involve its activity, the employing agency may advance or pay the travel expenses of the employee, and later obtain reimbursement from the agency properly chargeable with the travel expenses. An employee as defined by section 2105 of this title (except an individual whose pay is disbursed by the Secretary of the Senate or the Chief Administrative Officer of the House of Representatives) summoned, or assigned by his agency, to testify in his official capacity or produce official records, on behalf of a party other than the United States, is entitled to travel expenses under subchapter I of this chapter, except to the extent that travel expenses are paid to the employee for his appearance by the court, authority, or party which caused him to be summoned.