After making such investigation, if the fire marshal or deputy fire marshal is of the opinion that there is probable cause to believe a person guilty of the crime of arson, or of an attempt to commit the crime of arson, or of a conspiracy to defraud, or any criminal conduct in connection with such fires, he or she shall furnish to the state's attorney a copy of all such testimony together with the names and residences of witnesses and all necessary information obtained by him or her. Such state's attorney shall take such action as the facts justify and diligently inquire into all circumstances attending the fire. The fire marshal may assist in the prosecution either in person or by the deputy fire marshal.