28 U.S.C. § 514

When the head of an executive department or agency is of the opinion that the interests of the United States require the service of counsel on the examination of any witness concerning any claim, or on the legal investigation of any claim, pending in the department or agency, he shall notify the Attorney General, giving all facts necessary to enable him to furnish proper professional service in attending the examination or making the investigation, and the Attorney General shall provide for the service.