BANGKOK – A small but deadly fire at one of the Thai capital’s biggest shopping malls may have started in a basement room before spreading to an eighth-floor document room because of a bad vent, officials in Bangkok said Thursday.

City officials said at a news conference that the investigation of the blaze was still in its early stages and what caused the basement fire was not yet clear. They said smoke and burning matter from that blaze that was supposed to vent though a chimney may have backed up and started the upstairs fire.

Two people were killed in Wednesday’s fire and 20 were injured. The mall’s operating company, Central Pattana, said the two dead were employees who fell from windows trying to escape the smoke and flames.

The CentralWorld mall was closed Thursday for a cleanup but due to reopen Friday.

The mall complex also includes a hotel, convention centre and an office tower.

In response to concern that was raised about the mall’s evacuation, Central Pattana said in a statement that alarms had not been set off simultaneously in all areas of the building in order to try to avoid panic.

A shopper told The Associated Press on Wednesday night that he had heard alarms and announcements to leave the building.