In 1898 , the Spanish – American War increased the prison population from 26 to over 450 , and from 1905 to 1907 it was commanded by U.S. Army General George W. McIver . After the 1906 San Francisco earthquake , civilian prisoners were transferred to Alcatraz for safe confinement . On March 21 , 1907 , Alcatraz was officially designated as the Western U.S. Military Prison , later Pacific Branch , U.S. Disciplinary Barracks , 1915 . In 1909 construction began on the huge concrete main cell block , designed by Major Reuben Turner , which remains the island 's dominant feature . It was completed in 1912 . To accommodate the new cell block , the Citadel , a three-story barracks , was demolished down to the first floor , which was actually below ground level . The building had been constructed in an excavated pit creating a defensive dry moat . The first floor was then incorporated as a basement to the new cell block , giving rise to the popular legend of " dungeons " below the main cell block . The US Disciplinary Barracks was deactivated in October 1933 and transferred to the Bureau of Prisons
1906 San Francisco earthquake
1906 San Francisco earthquake
Alcatraz Island