Updated at: 10/17/2013 10:35 AM
(AP) LOS ANGELES - The Ventura County Fire Department says an investigation by the California fire marshal found that a Fourth of July fireworks accident in the city of Simi Valley was caused by the malfunction of a shell inside its mortar tube.
The blast during the aerial display show injured 28 people as debris from fireworks, mortar tubes and the racks holding them flew into the crowd.
The findings released Thursday conclude that a 5-inch diameter, two-part fireworks shell failed to function as designed and exploded inside a mortar tube, resulting in several subsequent sympathetic explosions.
The report found the show had proper permits, and Ventura County Fire Chief Mark Lorenzen says the display was properly designed and set up.
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