The City Commission may investigate the financial transaction of any office or department of the City government, and the acts and conduct of any official or employee. In conducting such investigation, the Commission may compel the attendance of witnesses, administer oaths and compel the production of books, papers, and other evidence, and for that purpose may issue subpoenas or attachments which shall be signed by the Mayor; which may be served and executed by any officer authorized by law To serve subpoenas or other process, or any peace officer of the City. If any witness shall refuse to appear or to testify to any facts within his knowledge, or to produce any papers or books in his possession, or under his control, relating to the matter under investigation before the Commission, such act shall constitute a misdemeanor and shall be punishable by a fine not to exceed $200.00. No witness shall be excluded from testifying, touching his knowledge of the matter under investigation in any such inquiry. But such testimony shall not be used against him in any criminal prosecution except for perjury committed upon such inquiry.