The Department of Explosives investigated the gas cylinder blast in Khagrachhari where seven people were injured.
A special team from the department's Chittagong office visited the site on Wednesday.
The blast took place at a gas cylinder shop at Khobongpuria area in the Sadar Upazila on Tuesday. Four of the critically injured victims have been transferred to Chittagong Medical College Hospital.
"It is not a normal explosion," said Explosives Inspector Md Tofazzal Hossain, also the team leader.
"Gas accumulated in the room when a smaller cylinder was being refilled from a bigger one," he explained, adding: "And then, there was an electric short-circuit when a switch was turned on, which triggered the explosion."
Tofazzal said Kanton Chakma, the shop's proprietor, only had permission to store and sell gas cylinders but he was illegally refilling smaller cylinders to line his pockets.
The blast took place during one such refill.
Tofazzal was accompanied by Assistant Explosives Inspector Md Mehedi Islam Khan and Khagrachhari police station's Sub-Inspector (SI) Bayesul Islam.
Shahadat Hossain Titu, officer-in-charge of Khagrachhari Sadar police station, said the team will submit its report by Sunday. "We will take further steps based on the investigators' recommendations," he said.
