This primary source comes from the Records of the Office of the Chief Signal Officer.
National Archives Identifier:
530504Full Citation: Slave Pen-Interior. Alexandria, Virginia; 1921; Mathew Brady Photographs of Civil War-Era Personalities and Scenes, 1921 - 1940; Records of the Office of the Chief Signal Officer, Record Group 111. [Online Version, https://docsteach.org/documents/document/slave-peninterior-alexandria-virginia, October 13, 2024]