28 C.F.R. § 76.14
Separation of functions
An employee or an agent of the Department who is or was engaged in investigative or prosecutive functions for or on behalf of the United States in a case may not participate in the decision of that case.
An employee or an agent of the Department who is or was engaged in investigative or prosecutive functions for or on behalf of the United States in a case may not participate in the decision of that case.