5 U.S.C. § 3106: Employment of attorneys; restrictions

Except as otherwise authorized by law, the head of an Executive department or military department may not employ an attorney or counsel for the conduct of litigation in which the United States, an agency, or employee thereof is a party, or is interested, or for the securing of evidence therefor, but shall refer the matter to the Department of Justice. This section does not apply to the employment and payment of counsel under section 1037 of title 10.