OAKLAND (CBS SF) — Three westbound lanes on the San Francisco-Oakland Bay Bridge were blocked early Thursday after two cars racing across the structure triggered a crash involving a box truck and two other vehicles, according to the California Highway Patrol.

The crash was first reported at 5:07 a.m. just west of Treasure Island. Impact of the collision spread debris across the upper deck, forcing the shutdown of three westbound lanes.

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At 6:21 a.m., the commute across the bridge was taking nearly an hour from the MacArthur Maze to San Francisco with traffic crawling along at 4 mph.

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At 6:30 a.m., all lanes were reopened but the massive backup heading into San Francisco Bay would take hours to clear. The CHP was advising drivers to take alternate routes across the Bay by taking the San Mateo Bridge or the Richmond-San Rafael Bridge.

Investigators said two cars were racing across the bridge and caused a collision involving a box truck with two other vehicles. The cars involved in the street racing reportedly fled the scene.

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There was no immediate word on injuries or any other details about the crash.