SIMI VALLEY, Calif. (AP) — More than two dozen people were injured when a wood platform holding live fireworks tipped over, sending the pyrotechnics into a crowd at a Fourth of July show northwest of Los Angeles, authorities said today.

Between 8,000 and 10,000 revelers were settling into their seats for the fireworks extravaganza at a Simi Valley park Thursday night when the spectacle of lights that were supposed to be launched in the sky never took off. Instead, a bright plume of red and white bursts spread across the ground, injuring 28 people and sending others fleeing for safety.

Police were still investigating what led to the explosion, but they said early indications show a platform designed and built to hold the fireworks gave way.

“For some unknown reasons the structure that holds these ordinances collapsed and caused them to be firing into the crowd,” Simi Valley police Cmdr. Stephanie Shannon.

Four people were listed in serious condition, but their injuries were not considered life-threatening. Sixteen more were taken to hospitals with minor to moderate injuries. The remainder were treated at the park where emergency crews, already on hand as a safety precaution, set up a triage area.