SIMI VALLEY, Calif. -

The Office of the State Fire Marshal released the official cause of a firework show explosion that injured 28 people on the Fourth of July.

The State Fire Marshal took over the investigation at the request of the Ventura County Fire Department.

The report states that the explosion was the result of a malfunction. The cause was determined to be a 5-inch diameter, two-part fireworks display shell that failed to work as designed. The shell went off inside the mortar tube causing an explosion and several other explosions.

The debris from the fireworks, the racks holding the fireworks and the mortar tubes flew in all directions at the launch site, injuring 28 people.

The report also states the the Simi Valley Police Department, Ventura County Fire Department and Cal Fire ruled out criminal activity or intent as a cause of the explosion.

The fire marshal also determined the show was properly permitted.