SANTA ANA, Calif. >> A fireworks company on Monday said bad fireworks appear to have caused a blast that injured more than three dozen people at a show in Southern California and also triggered problems at another one of its July 4 events.

Dennis Brady Jr., chief executive of Bethpage, N.Y.-based Bay Fireworks, said an apparent shell malfunction in Laguna Hills, Calif., however, did not send fireworks flying toward the crowd.

"We did experience a similar shell malfunction at Laguna Hills using the same product," Brady wrote in an email to the Associated Press. "Thankfully we did not experience the same chain reaction."

Both incidents appear to have been caused by a shell malfunction, he said, but the company is still waiting for official findings to be released.

State fire officials are investigating the cause of the accident in Simi Valley northwest of Los Angeles, which sent shrapnel flying across the field and wounded 39 people as many in the crowd of thousands fled for safety. Police initially said it appears a firework exploded prematurely in its mortar, knocking over others and aiming them across the field.

Pyrotechnic operator Bruce Sussin said inspectors checked the site before the show and everything was fine, until fireworks began setting each other off.

"Once that occurred and we realized we had a catastrophic failure, we immediately reached back and turned off the computer… the show immediately shut down," Sussin said.

In Simi Valley, the company had planned to shoot nearly 2,500 shells that ranged in size from 2.5 inches to 5 inches in diameter, Brady said. He said he did not know how many were shot toward the crowd.

All spectators were at least the required 350 feet away from the display, said Ventura County Fire spokesman Capt. Mike Lindbery.

The American Pyrotechnics Association said there were no immediate reports of widespread problems with a particular fireworks product over the holiday.

But troubles were reported at shows in the Southern California cities of San Juan Capistrano, Laguna Hills and Ojai, where a worker was injured. Although fireworks accidents at professional shows are rare, a worker was also injured Thursday in North Myrtle Beach, S.C., and a fireworks barge caught fire in a Montana lake at the start of the grand finale.

In Orange County, Laguna Hills City Manager Bruce Channing said their show, also operated by Bay, was shut down almost as soon as it started by a fire marshal because one of the mortars exploded at or near the ground, damaging some of the wiring to the rest of the fireworks that had yet to go off.

Channing said the city is seeking a refund and was told by Bay that it had received bad shells.

"We’d certainly like to have a better understanding of what occurred," he said. "I don’t think it’s ever expected that a firework will prematurely discharge… so when it happens there’s little other explanation to offer initially beyond bad product."

In Ojai, officials believe a shell malfunction also caused a firework to explode prematurely, said Doug Taylor, president and chief executive of New Castle, Pa.-based Zambelli Fireworks, which ran the show.

Taylor said a shell has two fuses — one on the outside that launches it into the air, and a second, timed fuse on the inside that causes it to burst open at the height of its trajectory, creating a colorful show. All shells are handmade, he said, and if they are not constructed properly the outside fuse can trigger the timed fuse prematurely.

Such accidents are extremely rare, however. Taylor’s company shoots more than 1 million shells a year and may have 10 bad shells among them.

Taylor said one of his first questions to the Chinese manufacturer of the malfunctioning shell will be if they also sell to Bay.