The probe into a teenage girl's "accidental" death after accepting a ride home at night in a lorry has intensified, with central police investigators joining local police in Ayutthaya.
The emphasis has switched to possible human trafficking, and her friend has been charged with criminal collusion.
Speaking after talking to doctors who examined the body of Nareekan "Ying" Yawiratch, 19, at the Central Institute of Forensic Science on Tuesday, deputy national police chief Srivara Ransibrahmanakul said it was quite clear that the police investigation was changing direction.
Earlier, police in Ayutthaya's Bang Pa-in district concluded that Nareekan died from injuries received when she fell from an articulated lorry after visiting an entertainment venue with friends on July 19.
The post mortem examination changed this finding.
"Evidence shows that Ying died because the right side of her head was hit with a hard, blunt object and not because of a fall from the semi-trailer," Pol Gen Srivara said.
Police from the Crime Suppression Division and the Anti-Trafficking in Persons Division had stepped in to find out if the case involved human trafficking, he said.
Pol Gen Srivara also said that he ordered a search for further evidence following a report the dead teenager's underpants were missing.
At Bang Pa-in police station on Tuesday, Nareekan's friend Sirinart Robrum, 18, was charged with colluding in detaining Nareekan in a way that led to her death.
Ms Sirinart denied she was a decoy and lured Nareekan into meeting a group of truck drivers. She said she and Nareekan had known lorry driver Surapol Darakham, 23, for two months. Nareekan had accepted a ride in his truck back to her dormitory because it was on his way.
Nareekan was later that night admitted to hospital with severe injuries to her head. It was reported Mr Surapol had sent her to hospital and claimed she had jumped off his truck. Nareekan died early in the morning.
Pol Maj Gen Maitree Chimcherd, commander of the Crime Suppression Division, said on Tuesday the Institute of Forensic Medicine would examine Nareekan's body to determine the exact cause of her death.
Investigators were looking for surveillance camera footage relating to the case. The truck driver, Mr Surapol, was being held in detention, he said.
A police source in Bang Pa-in said later on Tuesday that detectives searched Mr Surapol's truck and seized a 30-centimetre-long steel hammer and a steel wrench of the same size as possible evidence.
The source said they were normal tools carried by lorry drivers.
The source also said Nareekan's close friend had taken the missing underwear from Karunvej Ayutthaya Hospital after the victim was admitted for treatment. Police were acquiring the missingclothing.
