SIMI VALLEY, Calif. -- As many as 10,000 Fourth of July revelers were settling into seats for the fireworks show at a Simi Valley park when a bright plume of red and white bursts spread across the ground, injuring 28 people and sending others fleeing.
Bethpage-based Bay Fireworks, which set up the planned 22-minute display, said a mortar shell exploded prematurely.
The blast caused the wood rack holding the mortar to tip over, which then knocked into other fireworks and set them off, said Dennis Brady Jr., Bay Fireworks' chief executive.
Friday, police in the city northwest of Los Angeles were still investigating the cause.
The mortars toppled like "dominoes," and one or more fired into a crowd of spectators some 800 feet away, police Cmdr. Blair Summey told the Los Angeles Times.
A view of the scene from a distance Friday morning showed groups of mortars held vertically in boxlike wooden structures on the ground. In front of them, a number of mortar tubes lay scattered.
Brady blamed the accident on a "factory deficiency" and said the fireworks were made in China. Cellphone videos captured them exploding in spheres of sparks close to the ground, with fleeing people screaming.
Victims ranged in age from 17 months to 78, police said. Four people had serious but not life-threatening injuries.
"We're kind of heartbroken about what went on," said Joseph Mastanduno, Bay Fireworks' chief financial officer. "This is just something we take so seriously -- making sure everything is safe and everybody has a great time. When something is out of control, it's like a punch in the gut."
Friday, blackened debris littered the ground. Huge chunks of shrapnel were scattered across the park, and the stand the fireworks had been on was sitting, charred, in the middle of a green lawn.
Investigators planned to fly over the scene to photograph it and examine the debris, police said.
In a statement, Bay Fireworks said it "deeply regrets" the injuries and plans a "thorough" investigation.
According to its website, the company has produced shows for NASA, Disney World and the New York Mets.
With Ellen Yan