The chief of the fire department or other appropriate official of any city or other governmental unit or political subdivision may be designated by the state fire marshal as a special deputy for his city, governmental unit, or political subdivision to serve without pay from the state. The special deputy investigator shall immediately investigate the origin and circumstances of any fire designated by the marshal and determine the cause of the fire so far as is practicable. If it appears to any special deputy that the fire is of culpable or suspicious origin he shall immediately notify the state fire marshal.