Now, obviously, fireworks are designed to go up 200 feet in the air and explode. And we realized immediately that something was terribly, terribly wrong."
After the incident, fireworks were still on the field. Authorities evacuated the field, and waited for the Ventura County Fire Department bomb squad to respond to deactivate the unexploded fireworks.
The fire department said the injured were transported to Simi Valley Hospital, Las Robles Regional Hospital and Holy Cross Hospital.
"We're still in the process of sorting out exactly what happened and treating the injured and evacuating them to area hospitals," John Parks, the watch commander at the Simi Valley police department, said late Thursday.
Many of the injured were wrapped up with gauze with what appear to be burn-like injuries, and were being brought into ambulances with IV bags.
Police said fireworks accidentally detonated during the display. Precautionary measures taken ahead of the fireworks display were key to a quick response after the incident, Parks said.
"We had about 40 police personnel on scene, as well as fire and ambulance people who were staged at the part of the fireworks production for crowd and traffic control and security. So the response was really rapid and the people that were injured were attended to very quickly," he said.
Bay Fireworks, the company that put on the display, said in a statement that it "deeply regrets that people were injured."
The company asked people injured in the accident to contact the Simi Valley Rotary Club to get the contact information for the Bay Fireworks' insurance company representative, a statement said.
Bay Fireworks said it plans to conduct a full investigation of the fire accident, according to the statement.
The Independence Day event was sponsored by the support of the Rancho Simi Recreation and Parks District.
ABC News' Alex Stone and Rebecca Lee contributed to this report.