Several organizations had general investigative functions during the Civil War. From the outbreak of the war until early in 1862 the Secretary of State, acting through U.S. marshals and other Government officials, caused the arrest and imprisonment of many persons suspected of engaging in treasonable or disloyal activities. By an Executive order in February 1862 the authority to make such arrests was transferred to the War Department.
The offices in the War Department concerned with the investigation of subversive activities were those of the Judge Advocate General and the Provost Marshal General. Although a Judge Advocate for the Army had been authorized as early as March 2, 1849 (9 Stat. 351), increased demands arising from the expanded military operations after the outbreak of the Civil War required the services of more than one judge advocate for the administration of military justice. By an act of Congress approved July 17, 1862 (12 Stat. 598), the President was given authority to appoint a Judge Advocate General and several subordinate judge advocates.
AGO General Order 140, September 24, 1862, announced the newly created post of Provost Marshal General. An act of Congress approved March 3, 1863 (12 Stat. 732), created the Provost Marshal General's Bureau and provided for the appointment of a provost marshal for each congressional district. The provost marshals were military police one of whose duties was to arrest deserters and who could also be assigned to detect subversive activities. In addition, the War Department employed special provost marshals, detectives, and other agents when necessary.