What was Alcatraz used as?

Explanation

Alcatraz is a prison and a small island in the San Franciso Bay. Originally, it was the American military which had a prison on the island, but it was later taken over by the Federal Prison Service. The prison is particularly 'known' for the period 1934-1963, when a range of famous criminals were housed there. These include Al Capone, Robert Franklin Stroud, James "Whitey" Bulger and Alvin "Creepy" Karpis. Several films have been made about the prison.