It is what caused the fire:
"Propane leak cited in food truck blast" AP July 03, 2014
PHILADELPHIA — A ‘‘significant’’ propane tank leak on a food truck created a vapor cloud that was ignited by cooking grills, causing a fiery explosion that critically injured five people, police said Wednesday.
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Officials say the La Parrillada Chapina truck was licensed to operate in the Feltonville neighborhood of north Philadelphia and was open for business at the time of the explosion....
Surveillance video from two businesses captured the explosion, followed by a huge fireball that engulfed the truck and set a utility pole on fire. Two cars driving by the explosion also were briefly engulfed.
The US Bureau of Alcohol, Tobacco, Firearms, and Explosives; Philadelphia police detectives; and the city fire marshal’s office were continuing to investigate whether foul play was involved, police said.
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