(AP) — Police are still investigating but they say it appears a fireworks accident in California happened when a platform holding the fireworks tipped over, sending them shooting into the crowd, injuring 28 people.

As many as 10,000 people were attending the annual 4th of July show in Simi Valley, northwest of Los Angeles. Officials say that instead of launching into the sky, a bright plume of red and white bursts spread across the ground.

Four people are hospitalized in serious condition, but their injuries are not life-threatening. Another 16 were taken to area hospitals with minor to moderate injuries. Others hurt were treated at the park.

Authorities estimate people were about 900 feet away from where the fireworks were being launched.

Officials say most people responded admirably and left in an orderly fashion.