United States Penitentiary, Alcatraz Island, also known simply as Alcatraz or The Rock, was a maximum security federal prison on Alcatraz Island, 1.25 miles off the coast of San Francisco, California, United States. The site of a fort since the 1850s, the main prison building was built in 1910–12 as a U.S. Army military prison. The United States Department of Justice acquired the United States Disciplinary Barracks, Pacific Branch, on Alcatraz on October 12, 1933.