49 C.F.R. § 31.14 – Separation of functions.

(a) The investigating official, the reviewing official, and any employee or agent of the authority who takes part in investigating, preparing, or presenting a particular case may not, in such case or a factually related case—
1. (1) Participate in the hearing as the ALJ;
2. (2) Participate or advise in the initial decision or the review of the initial decision by the authority head, except as a witness or a representative in public proceedings; or
3. (3) Make the collection of penalties and assessments under 31 U.S.C. 3806.
(b) The ALJ shall not be responsible to, or subject to the supervision or direction of, the investigating official or the reviewing official.
(c) Except as provided in paragraph (a) of this section, the representative for the Government may be employed anywhere in the authority, including in the offices of either the investigating official or the reviewing official.