The sheriffs in their respective counties, whenever directed to do so in writing by the district attorney or by the Attorney General or Governor, shall make special investigation of any alleged violation of the law in their counties and shall prepare a written report setting forth what information has been obtained as a result of such investigation together with the names of such witnesses as have been secured with a summary of what can be proven by such witnesses, which report shall promptly after its preparation be presented to the official who directed the investigation and, if such official shall be the Governor or Attorney General, he may present it to any solicitor prosecuting criminal cases in the county. The sheriff of the county shall proceed promptly by himself or by a competent deputy of experience and fidelity to make such investigation when directed as aforesaid.