Section 2. It shall be the duty of the fire marshal or his assistants to examine into the cause, circumstances or origin of every fire occurring in any such city, by which any building, vessel, vehicle, or any personal property shall be burned, destroyed or damaged, wholly or partially; and inquire whether the fire was the result of carelessness or the act of an incendiary. Such fire marshall or his assistants may take the testimony, under oath, of all persons supposed to have any knowledge of the matter, reduce it to writing, and transmit the same to the mayor, with his report thereon; and shall report to the district attorney the facts he may have ascertained from such investigation when, in his opinion, there is evidence of arson, cause such person or persons to be arrested, and furnish to the district attorney the evidence with the names of witnesses.