40 C.F.R. § 24.09
The Presiding Officer shall be either the Regional Judicial Officer (as described in 40 CFR 22.04(b) ) or another attorney employed by the Agency, who has had no prior connection with the case, including the performance of any investigative or prosecuting functions. At no time after issuance of the initial administrative order and prior to issuance of the final order shall the Regional Administrator, Presiding Officer, or any person who will advise these officials in the decision on the case, discuss ex parte the merits of the proceeding with any interested person outside the Agency, with any Agency staff member who performs a prosecutorial or investigative function in such proceeding or a factually related proceeding, or with any representative of such person. If, after issuance of the initial order and prior to issuance of the final order, the Regional Administrator, Presiding Officer, or any person who will advise these officials in the decision on the case receives from or on behalf of any party in an ex parte communication information which is relevant to the decision on the case and to which other parties have not had an opportunity to respond, a summary of such information shall be served on all other parties, who shall have an opportunity to reply to same within ten (10) days of service of the summary.
40 C.F.R. §24.09