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 (2.01 km) 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. The island became adapted and used as a prison of the Federal Bureau of Prisons in August 1934 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 most secure prison.