At least 18 people have died in Bangladesh following an explosion at a mosque in Narayanganj district on Friday, the Dhaka Tribune reported. Twenty others are still undergoing treatment at the Sheikh Hasina National Institute of Burn and Plastic Surgery in Dhaka. Those undergoing treatment are not yet out of danger, doctors said.

The blast occurred around 9 pm on Friday when worshippers were at evening prayers at the Baitus Salah Jame Mosque. A ball of flames went through the mosque following the blast, according to witnesses. Authorities suspect that all six air conditioners on the ground floor of the mosque exploded.

“A pipeline of Titas Gas passes beneath the mosque,” Narayanganj Fire Service’s Deputy Assistant Director Abdullah Al Arefin said. “We are suspecting that gas had leaked from the pipeline and accumulated inside as the windows were closed. The explosion was probably triggered due to sparks when someone tried to switch the ACs or fans on or off.”

Fire services, the police and the gas company have all begun independent investigations into the blast.