The city commission, or any committee thereof, the city manager and any advisory board appointed by the commission for such purpose, shall have power at any time to cause the affairs of any department or the conduct of any officer or employee to be investigated; and for such purpose shall have power to compel the attendance of witnesses and the production of books, papers and other evidence; and for that purpose may issue subpoenas or attachments which shall be signed by the president or chairman of the body or by the officer making the investigation, and shall be served by any officer authorized by law to serve such process. The authority making such investigation shall also have power to cause the testimony to be given under oath to be administered by some officer authorized by general law to administer oaths; and shall also have power to punish as for contempt any person refusing to testify to any fact within his knowledge, or to produce any books or papers under his control, relating to the matter under investigation.