Bangkok city officials said the fire at the Rong Beer Na Ladprao bar in a northern part of the Thai capital broke out shortly before midnight on Sunday, and it took about 30 minutes for firefighters to bring it under control.
The bar, which in Thai calls itself a brewery or beer hall, featured live music and claimed to accommodate as many as 600 customers. It was not clear how many were there on Sunday night.
According to Bangkok's Erawan emergency services centre, the number of injured was 73, with 25 in critical condition. The Bangkok city government on Monday evening said there were 28 dead.
The site had been cordoned off by daybreak on Monday as dozens of Thai forensic officers combed for clues about what caused the fire. Associated Press journalists could see the bar's street-facing windows had been blown out and debris littering the footpath, including charred television sets, speakers and an electric guitar.
The scale of the devastation inside was visible through the shattered windows, with burned-out tables still holding empty beer bottles.
The dead had been trapped in bathrooms where they sheltered.
Thai national police chief Kittharath Punpetch said most of the dead were found trapped in windowless bathrooms near one of the rear exits of the bar, where they may have sheltered to escape the flames in the hall.
He said the exit had not been used, and people may have been obstructed from reaching it by a table set up in the hall to sell candy, or because it was too dark to find it.
Access to another exit near the kitchen might also have been narrowed by shelving units and lockers, said Kittharath, who visited the scene Monday morning. There were signs that at least some of the exit doors might have been locked shut, he added.
Investigators are focusing on the ceiling above the performance stage, where they found materials that may have been used as decorative elements, he said. Police will examine whether flammable materials were used in the interior and how electrical wiring was installed across the ceiling.
Several Buddhist monks visited the site on Monday morning to pray for the victims, while nurses handed out face masks to people nearby to help protect them from smoke and lingering fumes from the burned-out building.
A registration spot was set up to gather information from relatives coming to the scene looking for their loved ones.
A statement from the bar posted on Facebook expressed apologies and condolences and said it was fully cooperating with investigators. It said the bar's owner sustained serious injuries in the fire and was in a hospital intensive care unit.
Distressed family members gathered at Bangkok's Institute of Forensic Medicine in the afternoon to identify the bodies of the victims.
In 2022, 14 people were killed by a fire at a music bar in the eastern part of the country.
And more than a decade before that, 67 people were killed and more than 200 injured in a fire during a January 1, 2009, New Year's Eve celebration at the Santika nightclub in Thailand's capital. That blaze was apparently sparked by an indoor fireworks display.
