45 C.F.R. § 81.113
Only persons employed by or assigned to work with the reviewing authority who perform no investigative or prosecuting function in connection with a proceeding shall communicate ex parte with the reviewing authority, or the presiding officer, or any employee or person involved in the decisional process in such proceedings with respect to the merits of that or a factually related proceeding. The reviewing authority, the presiding officer, or any employee or person involved in the decisional process of a proceeding shall communicate ex parte with respect to the merits of that or a factually related proceeding only with persons employed by or assigned to work with them and who perform no investigative or prosecuting function in connection with the proceeding.
45 C.F.R. § 81.113