California officials said at least 19 people were injured Friday in Oakland after a building where they were working partially collapsed.

It happened as employees were pouring wet concrete from the second floor of the building, KTVU reported. The workers were briefly trapped in the wet concrete and some were left dangling off the scaffolding.

Emergency crews transported the injured from the site to Alta Bates Summit Medical Center and two other hospitals, a hospital spokesman said.

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No deaths or serious injuries were reported, Oakland Fire Battalion Chief Ian McWhorter said. The workers suffered from minor injuries, including scrapes and bruises.

The cause of the collapse is being investigated.

"We don't know exactly what caused this incident to occur," McWhorter said.

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The Associated Press contributed to this report.