In each and every county of the first class of this Commonwealth, the district attorney may appoint a chief county detective, an assistant chief county detective, and special county detectives not exceeding twenty in number, whose duties it shall be to investigate and make report to the district attorney as to the conduct in office of magistrates, constables, deputy constables, and other officers connected with the administration of criminal justice; to make such investigation and endeavor to obtain such evidence as may be required by the district attorney in any criminal case; and perform such other duties as the district attorney may direct. Said detectives shall be general police officers, and shall have all powers now conferred on constables by existing laws of this Commonwealth so far as they relate to crimes or criminal procedure.