CORONA, Calif. (AP) — Nine construction workers were injured — three critically — after a partial bridge collapse at a Southern California freeway on-ramp, authorities said.

The bridge deck was being lowered into place Friday night along State Route 91 in Corona when something went wrong during the jacking operation, according to the Riverside County Transportation Commission. The bridge deck suddenly dropped about 16 inches, striking wooden support beams, which hit the workers, officials said.

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All nine injured workers were hospitalized. One was released and three remained in critical condition Saturday, according to the Press-Enterprise newspaper. The nature of the injuries weren’t immediately known.

The freeway remained open but East Grand Boulevard was closed.

As of Saturday morning, “the bridge is seated in the proper position and inspections of the bridge have been made by the project structural engineering team,” the transportation commission said in a statement.

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The construction project began in 2014 as part of a widening of State Route 91.

The injured workers are affiliated with the prime contractor Atkinson Walsh Construction Joint Venture, the transportation commission said.

The California Occupation Safety and Health Administration will investigate the accident.

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