Eight people were killed and seven injured Sunday after a chemical accident at Siam Commercial Bank in the Thai capital of Bangkok. The incident, which the bank suspects was due to negligence, occurred when a fire prevention system upgrade went wrong and a chemical in the system sucked oxygen out from the building.

The incident originated in the basement of the bank when workers accidentally turned on the fire-extinguishing system while working on it, Channel News Asia reported, citing the police as telling the bank. The system reportedly contained the fire-fighting agent Pyrogen, which depletes oxygen in an area in order to curb the fire. Local media reported thick smoke coming out of the basement of the bank’s head office on Ratchadaphisek Road while the rescue operation was delayed due to its security system.

Five people suffocated to death at the scene while three others died in the hospital. The bank advises against the use of the chemical in the aerosol in rooms that are occupied and says, according to Channel News Asia, that “accidental exposure should be limited to five minutes.”

Vichit Suraphongchai, chairman of the executive committee of SCB, said, according to Channel News Asia: “The fire protection system's duty is to release a type of gas that causes the insufficiency of oxygen, which will prevent fire outbreaks from flaring,” adding: “But in this incident, there is people inside and lacks oxygen, so people suffocated, resulting in deaths and injuries. Therefore the incident is not caused by a fire or any explosion.”

The bank released a statement Monday saying that there was no fire or explosion at the bank during the incident. The statement added that the seven injured people were being treated in hospitals. Police officials and other concerned authorities are investigating the incident while the bank announced that the headquarters were operating as “normal.”

“From the preliminary investigation by the police, the incident may have been caused by the negligence of contractors who were hired to make additional improvements to the building's fire security system,” the bank said, adding: “The Bank would like to express our deep regret for those affected, especially those injured and those who lost their lives during this unfortunate incident. The Bank will provide initial assistance as deemed appropriate.”