Eve Bunting. Someone is Hiding on Alcatraz Island (1984) 136 pp.
Publisher’s description: “When Danny Sullivan saves an old woman from being mugged on a San Francisco street, he doesn’t know that the mugger is Priest’s brother. Priest is a member of the Outlaws, the toughest gang in Danny’s school. Now the Outlaws want revenge. Danny tries to escape by jumping on an excursion boat that’s leaving for Alcatraz Island where the old abandoned prison still stands. But the island doesn’t hold safety for Danny. It only holds terror since the Outlaws follow.” Juvenile.
Setting: Alcatraz Island, San Francisco