More than three dozen people were injured when a wood platform holding live fireworks tipped over, sending the pyrotechnics into a crowd at an Independence Day show outside Los Angeles, authorities said.

Between 8,000 and 10,000 revellers were settling into their seats for the fireworks extravaganza at a Simi Valley park on 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, sending onlookers fleeing for safety.

Thirty-nine people were injured. The victims, from 17 months to 78 years old, had burns and shrapnel wounds, and some were trampled, authorities and hospital officials said.

The injured included 12 children.

Only three remained hospitalized on Friday night.

Police were still investigating what caused Thursday night’s explosion, but it appeared a firework detonated prematurely in its mortar, knocking over a row of others, police said.

The mortars toppled like “dominoes” and one or more fired into a crowd of spectators some 800 feet (243 meters) away, police Cmdr. Blair Summey told the Los Angeles Times.

“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.

Meanwhile, a worker at a fireworks show in North Myrtle Beach, South Carolina, was injured on Thursday evening when a shell exploded prematurely. The explosion at the Cherry Grove Pier caused the show to end early, after just six minutes, and left a hole in the pier.

In California, a video clip aired on KCAL-TV shows a pair of firework blasts at or near the ground. Another clip, posted on YouTube and shot from a distance, shows three ground-level bursts. The fireworks continue for almost another minute before stopping.

“There was a big boom, everybody started running down the street, people were screaming,” Justice Allen, 17, of Simi Valley told the Los Angeles Times. “Everybody was just terrified. People hid in bushes.”

Authorities estimate people were 275 meters away from where the fireworks were being launched. One police officer who ran into the crowd when the blasts occurred had shrapnel tear through his leather belt and his clothing, Shannon said.

He had minor injuries to his back.

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