The commission shall investigate the enforcement of this Division 1 and its rules, and the action of the examiners herein provided for, and the conduct and action of the appointees in the classified service in its municipality. The commission may inquire as to the nature, tenure and compensation of all offices and places in the public service thereof. In the course of such investigations each commissioner may administer oaths. The commission may secure by its subpoena both the attendance and testimony of witnesses and the production of books and papers relevant to such investigations.