BANGKOK (AP) — Police in Thailand on Friday said they have filed criminal charges against a bus operator in connection with a bus fire that killed 23 young students and teachers on a school trip earlier this week, as investigations suggested a gas leak may have caused the blaze.

The owner of the bus, identified by police only by her first name, Panissara, was charged with negligence causing death, according to a statement from Pathum Thani Provincial Police.

Public outrage over the tragedy has put officials under scrutiny over safety standards after information emerged that the bus had passed an inspection about four months before the fire.

Six teachers and 39 elementary and junior high school students were on the bus when it caught fire Tuesday on a highway in Pathum Thani, a northern suburb of Bangkok. It spread so quickly that only 22 people were able to escape.

Funeral services for the dead started Thursday in their hometown of Lan Sak in the central province of Uthai Thani and were attended by high-ranking government officials and grief-stricken relatives. A royally sponsored cremation will take place next week.

Families of the victims have called for a tightening of vehicle safety. Inadequate enforcement of vehicular and road safety standards contributes to thousands of deaths each year in Thailand, which has one of the highest rates of traffic fatalities in the world.

Investigations into the fire have suggested there might have been a gas leak on the bus, which was fitted with 11 natural gas canisters although it had a permit for only six, Trairong Phiwpan, the head of the police forensics department, said Thursday. He said a gas tube had come loose, but it wasn’t clear if that happened before or after the accident.

He said investigators are still looking into what caused the sparks that ignited the blaze.

Many Thai vehicles run on compressed natural gas to save money. Officials have said that the bus, which was more than 50 years old, had been modified to run on CNG.

Police have said the driver, who has been charged with reckless driving and failing to stop to help others, claimed that a front tire had malfunctioned, making the bus bump into a car before skidding along a concrete highway barrier.

The Department of Land Transport presented details from its inspection of the bus at a meeting of Parliament's transportation committee on Thursday.

The gas tube on the bus that had come loose was from one of the five canisters that were not registered, said Cheep Nomsian, director of the Automotive Engineering Bureau.

He said the emergency exit appeared to be functioning and there was no indication of any tire rupture, which was initially thought to have contributed to the fire. He said, however, that the front axle of the bus was broken.

Additional complaints have also been filed against the bus company, which already had its operating license suspended, by the Department of Land Transport.

The department's director-general, Chirute Visalachitra, said at a news conference Thursday that the bus company, Chinnaboot Tour, had failed to respond to an urgent call for an inspection of its remaining buses. After using a GPS tracker, the department found the buses in a repair shop in northeastern Nakhon Ratchasima province, about 200 kilometers (125 miles) from Lopburi province, where the company had been ordered to send the buses, he said.

Chirute said the department ordered the seizure of the buses after it found that gas canisters had already been removed from them.

“This kind of action is a criminal offense, a serious violation of the law,” he said.