The commissioner may hold public hearings for the purpose of investigating any of the matters the commissioner is authorized to investigate under the provisions of this chapter. Such hearings must be held at such times and places as the commissioner deems fit. Any person interested in a matter under investigation may appear and testify at any public hearing held thereon. The commissioner or the commissioner’s duly authorized representative may subpoena and compel the attendance of any witness at any public hearing or at any session of any conference called and held as provided in this chapter and may administer an oath to any witness who is to testify thereat. A witness subpoenaed by the commissioner or by the commissioner’s representative must be paid the same mileage and per diem as are paid to witnesses in civil cases before the district court.