Siam Commercial Bank says seven other people were injured in the accident Sunday night.
The bank said in a statement Monday that the accident was believed to have been caused by "negligence on the part of our contractors hired to improve fire protection of the building."
It said workers activated a system that released pyrogen aerosol, which is normally intended to deprive fire of oxygen.
The bank identified those who died as a security guard and seven contractors. It said police were still investigating what went wrong.
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