The Alcatraz Federal Penitentiary or United States Penitentiary, Alcatraz Island 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 1910–1912 as a United States Army military prison.
The United States Department of Justice acquired the United States Disciplinary Barracks, Pacific Branch, on Alcatraz on 12 October 1933. The island became adapted and used as a prison of the Federal Bureau of Prisons in August 1934m after the buildings were modernized and security increased. Given this high security and the island's location in the cold waters and strong currents of San Francisco Bay, prison operators believed Alcatraz to be escape-proof and America's strongest prison.
The three-story cellhouse included the four main cell blocks – A-block through D-block, the warden's office, visitation room, the library, and the barber shop. The prison cells typically measured 9 feet by 5 feet and 7 feet high. The cells were primitive and lacked privacy.