49 CFR §390.8

1. (a)An Agency employee who has taken an active part in investigating, prosecuting, advocating, or making an initial Agency determination in a proceeding under § 380.723, § 382.717, § 390.115, § 390.135, or § 391.47 of this chapter or section 5.4 to appendix A to subpart B of part 395 of this chapter may not, in that case or a factually-related case, advise or assist the Agency official authorized to issue a final decision in the applicable proceeding.
2. (b)Nothing in this section shall preclude the Agency official authorized to issue a final decision or anyone advising that Agency official from taking part in a determination to launch an investigation or issue a complaint, or similar preliminary decision.