The Alcatraz Federal Penitentiary or United States Penitentiary, Alcatraz Island (often just referred to as Alcatraz) was a maximum high-security Federal prison on Alcatraz Island, 1.25 miles (2.01 km) off the coast of San Francisco, California, USA, which operated from 1934 to 1963.The main prison building was built in 1910–12 during its time as a United States Army military prison; Alcatraz had been the site of a citadel since the 1860s.