Upon the written demand of a judge of the district court, with or without the consent and approval of the state‘s attorney of the county wherein such duties are to be performed, the attorney general, either personally or through the attorney general’s assistants, shall be required to make a full and complete investigation of any criminal matter or complaint referred to in the demand. The attorney general shall take full charge of and shall conduct any criminal prosecution in any county within the district of said district judge to the same effect and with like power and authority as the duly elected state’s attorney of that county. All expenses, including mileage as now provided by law for state officers, and disbursements for subsistence while performing those duties incurred by the attorney general, must be paid and allowed by the county in which the said duties were performed in the manner in which claims against the county are allowed and paid, after an itemized statement thereof has been approved by the judge who requested that the same be performed.