Courts of inquiry, to consist of one or more officers, may be, or on the request of the officer involved, shall be, instituted by the Adjutant General for the purpose of investigating the conduct of any officer, any accusation or imputation against him, or any acts made the subject of military complaint. Such court of inquiry shall, without delay, report a statement of facts, and, when required, the evidence adduced and an opinion thereon to the Adjutant General, who may, in his discretion, thereupon order a court-martial or efficiency board for the trial of the officer whose conduct has been inquired into.