"For spectators, this is incredibly rare," Heckman said. "This is just one that is going to stop everybody in their tracks and say, 'We've got to figure out what happened.'"
The accident will undoubtedly prompt members of the National Fire Protection Association, which develops the codes that guide the industry, to evaluate whether changes are needed, Heckman added.
Bay Fireworks is licensed by the state and had no violations on their record and the show did not require a state fire marshal permit, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection. The state could cite the company after reviewing the results of the investigation by Ventura County, which issued the permit for the event.
Cellphone videos captured a frantic scene in Simi Valley. Fireworks exploded in spheres of sparks close to the ground, and smoke enveloped the park grounds. People screamed and ran as one man could be heard mistakenly shouting that someone was shooting.
Colette Schmidt was watching with guests at her home across the street when it became clear something went terribly wrong. After a few fireworks lofted perfectly in the sky, there was a big explosion on the ground and a volley of blasts.
One landed and left a crater across from her home, then bounced twice and shot over nearby trees, exploding in a puff of reddish-purple smoke, said Schmidt's daughter, Alessi Smith.
The family herded their guests inside and drew the blinds as sparks and embers rained down, Schmidt said.
"It was terrible but we were so blessed because we had 150 people here and not one single spark hit our house," she said.
A bomb squad was sent to the park to help deactivate the remaining 60 percent of fireworks that weren't launched.
On Friday morning, blackened debris from the explosion littered the ground. Huge chunks of shrapnel were still scattered across the park and the boxes the mortars had been sitting in were left in the middle of a green field.
Authorities said investigators planned to examine the debris and fly over the scene to photograph it.
The annual July Fourth celebration has been sponsored by the city and the local Rotary Club since 1970.
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 because of an error in the computer system that sets off the pyrotechnics. No one was injured. That show was not produced by Bay Fireworks.
Heckman, with the fireworks trade group, said that while the investigation has not yet revealed the cause of the Simi Valley blast, she believes it was probably a product malfunction.
The industry takes such incidents seriously, especially when they involve spectators. She noted that Bay Fireworks has been in business for a long time and has done significant productions.
The company website says it has produced events for NASA, Walt Disney World and Legoland.
"This incident is a dark cloud over the entire industry," she said. "We don't take it lightly."
Associated Press writers Amy Taxin in Santa Ana, Calif., and Greg Risling and Shaya Mohajer in Los Angeles contributed to this report.