Officials in Thailand's capital say that a small but deadly fire at one of the city's biggest shopping malls may have started in a basement room before spreading to an eighth-floor document room because of a bad vent.

City officials said at a news conference Thursday that the investigation 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 CentralWorld mall was closed Thursday for a cleanup but due to reopen Friday.

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