Which island in San Francisco Bay was a notorious federal prison?

The island in San Francisco Bay that became a notorious federal prison was Alcatraz Island. Located about 1.25 miles offshore, the rocky island was isolated by cold water and strong tidal currents, conditions that helped create its fearsome reputation.

Alcatraz had already served as a military fort, lighthouse site and military prison before the U.S. Department of Justice converted it into a federal penitentiary in 1934. The prison held inmates considered especially difficult or dangerous, including famous criminals such as Al Capone and George “Machine Gun” Kelly. Its official name was United States Penitentiary, Alcatraz Island, though it was widely known as “The Rock.”

The federal prison closed on March 21, 1963, partly because operating the remote facility was expensive. The island later became a National Historic Landmark and a major tourist attraction. Alcatraz is sometimes confused with San Quentin, but San Quentin is a mainland prison in California, not an island in San Francisco Bay.