VALLEJO (CBS SF) – A three-alarm fire erupted early Tuesday in an abandoned Vallejo building frequented by squatters, authorities said.

In a Facebook post, Vallejo firefighters said the fire was in a downtown building near York St. and Sutter St.

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Authorities said an earlier fire had damaged the building.

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Arriving firefighters found the second floor fully engulfed. The blaze quickly spread throughout the building, triggering a third alarm.

The flames were quickly knocked down but the roof had collapsed and firefighters were forced to take a defensive approach.

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A cause of the blaze remained under investigation. No injuries were reported.