The closure began Monday and is scheduled to last through Friday, temporarily cutting off public access to the former prison site in the middle of San Francisco Bay.
In an alert posted Friday, the Park Service said access to the island is temporarily closed for dock repair. All scheduled tours during the closure period have been refunded, officials said.
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Alcatraz City Cruises, the island's official ferry operator, has suspended its trips to the island for the week and is directing visitors to alternative bay cruise options instead.
The National Park Service has not released further details about the repair work, but said Alcatraz is expected to reopen once the dock project is complete.