July 4 Fireworks Explosion Result of Malfunction, Fire Marshal Says

The California Fire Marshal's office has determined a fireworks explosion that injured 28 people at a Simi Valley July Fourth celebration was due to malfunction.

After an in-depth investigation, the fire mashall's office, or CAL FIRE, determined the cause of the blast was the failure of a five-inch diameter, two-part fireworks display shell to function as designed. This type of shell consists of an upper shell attached to a lower shell, a fuse and a lift charge. The investigation concluded that the shell went off inside a mortar tube resulting in an explosion and several subsequent sympathetic explosions.  

The explosions sent debris from the fireworks, the racks holding them, and the mortar tubes in all directions from the launch site, including into a nearby crowd.

A CAL FIRE report also indicates the Simi Valley Police Department has ruled out criminal activity or intent as a cause and a joint investigation by the Ventura County Fire Department and CAL FIRE determined that the show was properly permitted. 

Ventura County Fire Chief Mark Lorenzen echoed the findings, calling the incident "a tragic malfunction."

"The display was properly designed and set up. That being said, our responsibility is to do everything we can to ensure the safety of the people of Ventura County. As a result, we will be reviewing the state fireworks polices to see if additional local measures might be warranted," Lorenzen said.

See initial coverage of the incident here:

Simi Valley Fireworks Show Accident Injures 28 Onlookers

Update: Fireworks Mishap Investigation Continues

Fireworks Company Issues Statement, 'Deeply Regrets' Simi Valley Injuries