Any officer or employee engaged in the performance of investigative or prosecutorial functions for the Department in a proceeding as defined in § 501.702 may not, in that proceeding or one that is factually related, participate or advise in the decision pursuant to Section 557 of the Administrative Procedure Act, 5 U.S.C. 557 , except as a witness or counsel in the proceeding.
31 C.F.R. §501.720