Tests found no alcohol or drugs in the body of the 64-year-old driver of the passenger van that collided head-on with a pickup and exploded in Chon Buri on Monday, killing 25 people, police said on Wednesday.
The criminal investigation of the accident was now closed, it was announced.
Pol Maj Gen Pornchai Suteerakune, commander of the Institute of Forensic Medicine, said the autopsy found no trace of alcohol or drugs in Sumon Eiamsombat's blood or tissue.
Sumon was at the wheel of the passenger van travelling from Chanthaburi to Bangkok that swerved across the road, hit an oncoming pickup truck also loaded with passengers and burst into flames on a highway in Ban Bung district of Chon Buri. There were only two survivors.
Pol Maj Gen Pornchai said three of the victims had yet to be identified.
Pol Maj Gen Piyaphan Pingmuang, deputy police spokesman, said the criminal case arising from the collision was now closed because the driver who allegedly caused it was killed in the incident. However, relatives of the victims could file a civil lawsuit for compensation.
Deputy Prime Minister and Defence Minister Prawit Wongsuwon said the van belonged to the driver who was exhausted from making too many round trips. Cabinet had discussed the possibility of minibuses replacing vans on short-distance, interprovincial routes, he added.
It was earlier reported that Sumon was making his fifth 500 kilometre round trip between Bangkok and the eastern province of Chanthaburi in 31 hours on Sunday and Monday, both New Year holidays.