BANGKOK (AP) — Thai police on Wednesday said they were investigating whether the school bus fire that killed 23 young students and teachers in suburban Bangkok was caused by negligence after filing initial charges against the driver.

The fire on the bus carrying six teachers and 39 elementary and junior high school students on Tuesday spread so quickly many were unable to escape.

The driver, Saman Chanput, was arrested several hours later and charged with reckless driving causing deaths and injuries, failing to stop to help others and failing to report the accident, police said.

Authorities were investigating if the fire might be caused by negligence from both the driver and the bus company, and will press charges against all parties responsible, acting police chief Kitrat Phanphet told a news conference.

While an initial investigation suggested that the driver was not speeding, police found 11 natural gas canisters inside the bus that had a permit to install only six, Kitrat said.

Police have not officially concluded the cause of the fire, but they said the driver told investigators he was driving normally until the bus lost balance at its front tire, hit another car and scraped a concrete highway barrier. The sparks from the friction might have caught on the highly flammable gas canisters and ignited the blaze, police said.

Kitrat said the fact that the driver did not immediately stop after feeling the bus was losing balance could be grounds for negligence.

The inspection of the bus found that its emergency exit could be opened, but it wasn't clear if it worked properly, said chief of police forensics Trairong Phiwpan. He also said they did not find any window breakers.

In an interview with public broadcaster Thai PBS, bus company owner Songwit Chinnaboot said the vehicle was inspected for safety twice a year as required and that the gas cylinders had passed safety standards.

The families of the victims were driven from Uthai Thani, the central province where the bus departed from on a school trip, to Bangkok to provide DNA samples for the identification process.

Three students were hospitalized, two of them in serious condition. A 7-year-old girl suffered burns on her face, and a surgeon said doctors were trying to save her eyesight.

Thailand's Department of Land Transport said it was implementing urgent inspections of all natural gas-fueled buses. The department also will upgrade its safety guidelines to require crisis management training for drivers and safety inspection every time such vehicles are to be commissioned by schools, said Seksom Akraphand, the agency's deputy director-general.

He added that the department had suspended licenses for both the bus company and the driver.

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This story corrects location of Uthai Thani province.

Ambulances carrying bodies of victims of a school bus fire leave from the Police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024. (AP Photo/Sakchai Lalit)

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Nurse students pay respect to one of the victims of a school bus fire, at Police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024. (AP Photo/Sakchai Lalit)

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Rescuers gather at the site of a bus that caught fire, carrying young students with their teachers, in suburban Bangkok, Tuesday, Oct. 1, 2024. (AP Photo/Sakchai Lalit)

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Family members of a victim arrive at a forensic police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024, after a bus carrying students and teachers caught a fire Tuesday. (AP Photo/Sakchai Lalit)

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Family members of a victim arrive at a forensic police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024, after a bus carrying students and teachers caught a fire Tuesday. (AP Photo/Sakchai Lalit)

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Family members of a victim arrive at a forensic police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024. (AP Photo/Sakchai Lalit)

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Forensic officers stand beside empty coffins at a forensic police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024, following a deadly bus fire incident. (AP Photo/Sakchai Lalit)

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A rescuer inspects a bus that caught fire, carrying young students with their teachers, in suburban Bangkok, Tuesday, Oct. 1, 2024. (AP Photo/Sakchai Lalit)

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Nurse students pay respect to one of the victims of a school bus fire, at Police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024. (AP Photo/Sakchai Lalit)

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An ambulance carrying a body leaves from the Police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024, (AP Photo/Sakchai Lalit)

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A forensic officer stands behind the forensic department at a police hospital in Bangkok, Thailand, Wednesday, Oct. 2, 2024, as family members of victims visit there after a bus carrying students and teachers caught a fire Tuesday. (AP Photo/Sakchai Lalit)

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