A bus carrying young students and their teachers on a school trip caught fire in suburban Bangkok on Tuesday, leaving more than 20 feared dead, officials and rescuers said.

The bus with 45 passengers, six teachers and 39 elementary and junior high school students, was traveling from the central Uthai Thani province when it caught fire in Pathum Thani province, a northern suburb of Thailand’s capital, Acting Police Commissioner Kitrat Phanphet said.

The fire was first reported around noon and was put out less than an hour later, but rescuers said they could not get on board for hours as the heat inside the natural gas-fueled vehicle could have caused more explosions.

Police were still working to identify the dead but three teachers and 20 students remain unaccounted for, Kitrat said.

The cause of the blaze was not immediately known. Kitrat said the initial investigation indicates a tire had exploded and caused sparks, which ignited a blaze that spread through the bus. He did not elaborate.

No other vehicles were involved, he said.

There were discrepancies in reports on the number of the people aboard the bus. Rescuers cited teachers who survived as saying there were three buses from the school for this trip and that along the way, some students had moved to different buses from the ones they were initially on.

Videos posted on social media showed the entire bus engulfed in a fire with black smoke pouring out of the bus.

Piyalak Thinkaew, a rescuer from the Ruam Katanyu Foundation, told reporters that most of the bodies were found in the middle and back seats, leading them to assume the victims had moved back and that the fire had started at the front of the bus.

The police were looking for the driver who appeared to have fled the scene, Kitrat said, adding that the bus company and individuals involved may be charged if they are found responsible.

“Such an incident causes a great sorrow and grief,” he told reporters at a news conference.

“There is no way we will distort the fact or help anyone” escape justice, Kitrat said. He added that 16 students had been treated for minor injuries and later sent home while three others were hospitalized.

The nearby patRangsit Hospital said three girls were initially treated there, one with burns to the face, mouth and eyes. Surgeon Anocha Takham said the doctors would do their best to save the girl, who is around 7, from losing her sight.

The girls were later moved to other hospitals for further treatment.

Kitrat said a teacher who survived told police the fire had spread so quickly she didn’t even have the time to grab her mobile phone. Some on board managed to escape through the door while others jumped out through the windows.

Prime Minister Paetongtarn Shinawatra offered her condolences in a post on the social media platform X, promising the government would cover medical expenses and help the victims’ families. She later visited the injured in the hospital.

When reporters asked her about the fire at the Government House, Paetongtarn was overcome by emotion and burst into tears. She became prime minister in August and is the mother of two children.

The accident has prompted criticisms over the safety of children traveling long hours across provinces on roads notorious for their high rates of traffic accidents and deaths.

The World Health Organization estimates that every year, 20,000 people are killed and a million are injured in road accidents in Thailand.