The ATF National Response Team and investigative partners determined the explosion at a Chandler strip mall that left several people injured last week was accidental.
The findings determined that the explosion was caused by an unintentional natural gas leak ignited by an independent ignition source.
The investigation found no criminal activity and said the incident was accidental.
Chandler Fire Department crews responded to the scene around 9:45 a.m. near Rural and Ray roads on Thursday, Aug. 26, and found the roof collapsed and fire burning at a printing shop in the center.
Four people were inside at the time of the explosion and suffered serious injuries. All four are currently still receiving treatment.
Valleywise Health says the four victims suffered burn injuries to as much as 30% of their bodies.