The Service is authorized to make a full investigation of any complaint referred to it by the court under section 2000a–3(d) of this title and may hold such hearings with respect thereto as may be necessary. The Service shall conduct any hearings with respect to any such complaint in executive session, and shall not release any testimony given therein except by agreement of all parties involved in the complaint with the permission of the court, and the Service shall endeavor to bring about a voluntary settlement between the parties.