Police in the city northwest of Los Angeles 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 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 box-like wooden structures sitting on the ground. In front of them, a number of mortar tubes lay horizontally scattered on the ground.
"For some unknown reasons the structure that holds these ordinances collapsed and caused them to be firing into the crowd," said Simi Valley police Cmdr. Stephanie Shannon.
Cellphone videos captured fireworks exploding in spheres of sparks close to the ground, with smoke and people screaming.
The 28 victims ranged in age from 8 to 78 years old, Sgt. Tom Meyer said. A total of 20 people were taken by ambulance to area hospitals. Four suffered serious, but not life-threatening, injuries.
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.
On Friday morning, blackened debris from the explosion littered the ground. Huge chunks of black shrapnel were still 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, Meyer said.
The fireworks company, Bay Fireworks, said in a statement that it regrets the injuries, and planned a thorough investigation, with results to be made public.
The company also said that injured spectators should contact the Simi Valley Rotary Club to reach the company's insurer. The annual July Fourth celebration has been sponsored by the city and the local Rotary Club for the past 43 years.
The company, based in Bethpage, N.Y., has produced events for NASA, Walt Disney World, Legoland and the Republican National Convention, according to the company's website.
The mishap came a year after a fireworks show in San Diego exploded in about 20 seconds and sent multiple bulb-shaped explosions over the bay. The show's producer blamed a "technical glitch," saying an error in its computer system caused tens of thousands of fireworks on four barges to go off simultaneously with a single command.
That show was not produced by Bay Fireworks.
A video clip of the Simi Valley accident that 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.
Colette Schmidt, who lives across the street from the park and had about 150 guests over to watch the fireworks from their front lawn, said they could tell almost immediately that something went wrong.
Two regular fireworks went off, she said, and then others started exploding at ground level and shooting off horizontally toward the crowd "like a fan."
One hit the ground across from their home, leaving a crater, and bounced twice before shooting up over nearby trees and exploding in a puff of reddish-purple smoke, sad Schmidt's daughter, Alessi Smith.
The family herded their guests inside and drew the blinds as sparks and embers rained down. Sparks set a dry field next door on fire, but the flames were quickly put out.
"It was terrible but we were so blessed because we had 150 people here and not one single spark hit our house," she said. "It was all around us, but nothing here."
Shannon said most people responded admirably and left in an orderly fashion. A bomb squad was sent to the park to deactivate the remainder of the fireworks.
Shannon said the fireworks primarily shot in one direction.
"They are going to travel the same distance across the park as they would in the air," she said. "The ones that had actually ignited that had to run their fuse were going directly into the crowd."
Simi Valley, home to the Ronald Reagan Presidential Library, is about 40 miles northwest of Los Angeles.
Other fireworks mishaps in the U.S. Thursday:
- One person was killed and others injured after a pickup plowed into pedestrians after the fireworks show in Grand Lake, Colo. The 33-year-old driver was arrested and is being held on suspicion of driving under the influence as investigators piece together what happened. Colorado State Trooper Josh Lewis says initial reports indicated those hit were on the side of the road.
- A barge being used to set off the annual Fourth of July fireworks on Whitefish Lake in Montana caught fire just as the grand finale began. The local fire chief said a spark from a low-bursting firework started the blaze. The two pyrotechnicians on board dove into the water. No one was injured.
- A worker at a fireworks show in North Myrtle Beach, S.C., was injured 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 Seattle, fire investigators blamed illegal fireworks for a blaze that damaged 14 boats at a storage facility as the city's official fireworks display was exploding nearby, causing an estimated $1.5 million in damages. One firefighter received minor injuries fighting the fire.