SIMI VALLEY (CBSLA.com) — Officials say more than two dozen people were injured Thursday during a fireworks malfunction in Simi Valley.

A crowd of approximately 10,000 had gathered for the annual Rotary Club Independence Day fireworks show at Rancho Santa Susana Community Park, located in the 5000 block of Los Angeles Avenue, when several of the devices accidentally detonated shortly after 9 p.m.

The Simi Valley Police Department say 28 people were injured, 20 of whom were taken to hospitals. Four of those transported suffered moderate injuries, such as broken bones and shrapnel wounds, and 16 sustained minor injuries.

Those injured were initially treated at a makeshift triage area and later taken to Simi Valley Hospital, Los Robles Hospital and Medical Center and Providence Holy Cross Medical Center. Their ages ranged from eight to 78.

The misfire reportedly happened a couple minutes into the show. The event was quickly stopped and the crowd was told to leave the premises.

Spectator Zach Reister posted video of the incident on Youtube. The footage captures the explosion and a man saying, “That wasn’t supposed to happen.”

“It was almost as if one of the crates that had several mortars on it tipped over, and it just set off a chain reaction. There were things going into the crowd, they were shooting up into the air, there was smoke everywhere, panic started to settle in…” spectator Gabriel Pabon said.

Rancho Simi Recreation and Parks district ranger Steve Williams said that, as the show was just getting underway, some fireworks seemed to fire off to the side rather than launching vertically.

“I’ve never seen this big of a medical response in Simi,” Williams said.

Simi Valley resident Tim Klunen, 33, watched the event unfold from a hill above the show.

“About 30 seconds into the show, we saw that fireworks that were supposed to be shooting up high exploded on the ground,” Klunen said. “It appeared the fireworks were shooting off horizontally into the crowd and into the neighborhood. It was instantaneous, they were there already. It looked like the first people we saw running from the scene were the fireworks crew.”

Police say at least one of the firework launching stations fell over during the mishap.

Federal officials are now investigating the incident as an industrial accident. They do not suspect foul play.

A police bomb crew was called in to render the remaining undetonated fireworks safe. A pyrotechnics crew was scheduled to remove those devices Friday morning.