PLANTATION, Fla. — An open gas valve and ignition from an air conditioning unit are believed to have caused the massive explosion that injured 22 people in July at a South Florida strip mall.

The preliminary Origin and Cause report from the Florida Department of Financial Services found the gas entered the building "due to an open unrestricted gas line" at the vacant PizzaFire restaurant. 

Dash cam video from a mini van showed the explosion happening just before 11:30 a.m. July 6 at the shuttered restaurant at 1025 S. University Dr. in Plantation, Florida.

The report found "either the ball valve at the rear of the business or the ball valve on the top side (customer side) of the meter was turned on the day of the incident shortly after 7AM." With gas freely flowing for hours, investigators say it was ignited by an air conditioning unit that flipped on in the building.

It's not yet known how or why the gas turned on.

Debris was flung across a busy road, destroying the shuttered restaurant and damaging cars and other businesses. Concrete was thrown some 50 yards away and pieces of metal were reported to be scattered as far as 100 yards across the street.

Watch: Gas explosion at Florida shopping plaza injures 'multiple' people

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