SIMI VALLEY, Calif. >> When a July Fourth fireworks display exploded and sent red and white bursts into spectators at a Southern California park, Paulina Mulkern saw shrapnel headed straight for her 4-year-old cousin.

Mulkern pushed the girl under a lawn chair, and then shielded her 7-year-old cousin with her body as scorching debris flew overhead.

“You feel the big old heat come right over your back,” she said Friday, still shaking as she recounted the explosion the night before that left her hospitalized with bruises and red marks on her back.

Thirty-nine people were injured as many in the crowd of thousands fled for safety. 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 Friday night.

Mulkern said she went into shock after being hit by a flying piece of debris, trembling badly as she was carried to a road where rescuers stripped off most of her clothes and wrapped her in a blanket.

“I was really terrified. Every time someone launched a firework it got me into panic mode and they just told me, ignore the sounds around you and concentrate on your breathing,” she recalled.

Police in Simi Valley, northwest of Los Angeles, said it appeared a firework exploded prematurely in its mortar, knocking over others and aiming them across the field. Fire investigators, however, said later they had not yet determined a cause.

Police based their initial statement on the accounts of witnesses, who said a rack of fireworks fell over, said Ventura County Fire Capt. Mike Lindberry.

Among other key questions investigators were trying to answer was whether the pyrotechnics display was set far enough away from spectators, and even if all the rules were followed, whether those guidelines needed to be revised so that the public is kept farther back from launch sites.

Regulations require crowds be kept 70 feet away for every inch of diameter of the largest shell.

By those standards, spectators should have been at least 350 feet away from the show put on by Bethpage, N.Y.-based Bay Fireworks, said Ventura County Fire Department Deputy Chief Mike LaPlant. The distance of spectators from the show will be one of many factors considered by investigators.

“The distances were either at or beyond the normal distances, the prescribed distances, for that sized shell,” he said.

The company said it regretted that spectators were injured and that it planned to publicly release the results of a thorough investigation.

Of the victims, all but three had been treated and released by late Friday, hospital officials said.

One patient was being treated by specialists at Grossman Burn Center and two other adults remained hospitalized in fair condition, said Kim Milstein, chief executive of Simi Valley Hospital.

Although fireworks accidents at professional shows are rare, they are not unheard of. The blast in Simi Valley was itself among several mishaps nationwide Thursday, including errant explosions injuring workers at shows in nearby Ojai, as well as North Myrtle Beach, S.C., and a fireworks barge that caught fire in a Montana lake at the start of the grand finale.

In 2008, fireworks shells exploded on the ground and another one launched into the crowd, injuring five people at an event that also involved Bay Fireworks, said Julie L. Heckman, executive director of the American Pyrotechnics Association, a fireworks trade group. 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, she added.

“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 the figure out what happened.”’

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. “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.”