For many people gathered to watch July 4th fireworks at a Southern California park, it took time to realize that the wild chain of explosions was not just part of the show.

But those up close Thursday night knew immediately that something was wrong. They included Paulina Mulkern, who had to shove her 4-year-old cousin under a lawn chair as shrapnel came flying and then shielded a 7-year-old cousin with her body as scorching debris flew overhead.

“You feel the big old heat come right over your back,” Mulkern said Friday, still shaking a day after the chain reaction of accidental explosions at an annual fireworks show that had been put on since 1970 northwest of Los Angeles.

Thirty-nine people ranging in age from 17 months to 78 years old were injured. Some suffered burns and shrapnel wounds and some were trampled, authorities and hospital officials said. The injured included 12 children.

Three of the wounded remained hospitalized Friday night. One was being treated at a burn center in West Hills and two more were in fair condition at a Simi Valley hospital, officials said.

Mulkern said she went into shock after being hit by a flying piece of debris that left her with bruises and red marks on her back. Rescuers stripped off most of her clothes and wrapped her in a blanket.

“I was really terrified,” she said. “Every time someone launched a firework it got me into panic mode.”

Cellphone videos captured a frantic scene among the crowd of 10,000. Fireworks exploded in big balls of sparks close to the ground, and smoke enveloped the park grounds. People screamed and ran.

Police at first said it appeared that a firework exploded prematurely in its mortar, knocking over other pyrotechnics and aiming them across the field. Fire investigators, however, said later that 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 and whether guidelines needed to be revised to protect spectators. Regulations require crowds be kept 70 feet away for every inch of diameter of the largest shell. By those standards, the crowd should have been at least 350 feet away from the show, said Ventura County Fire Department Deputy Chief Mike LaPlant. He said all of the injuries occurred at a distance of 350 feet or more.

Bethpage, N.Y.-based Bay Fireworks, the company that put on the show, said it regretted that spectators were injured and would make public the results of a thorough investigation.

Fireworks accidents at professional shows are rare. In 2008 in New York, fireworks shells exploded on the ground and another launched into a 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.

Last year, a fireworks show in San Diego put on by a different firm went off in its entirety about 20 seconds into what was supposed to be a nearly 20-minute show, sending multiple explosions over the bay because of an error in the pyrotechnic computer system. No one was injured.

Thursday’s accident “is a dark cloud over the entire industry,” Heckman said. “We don’t take it lightly.”

— Associated Press

Amy Taxin, Greg Risling and Shaya Mohajer contributed to this report