Work was half complete in 1860. Construction continued into the American Civil War, but eventually stopped, and the fort was used as a military prison. It was here that Dr. Samuel Mudd and three others charged with conspiracy in the assassination of President Abraham Lincoln were imprisoned. Mudd was pardoned in 1869 after averting a viral outbreak. This use as a military prison continued until 1874. With the introduction of coal-fueled ships, the Dry Tortugas became a coaling station for U.S. Navy ships.