A couple in Roi Et province were left traumatised and wracked with doubt after the dead body of their 14-year-old son was found near the Roi Et-At Samat road in Thawatchaburi district on Feb 25 last year.
Local police later concluded that their son, Kritsada Sisong, had died in an accident. However, this failed to convince the parents who believed Kritsada had been murdered and then dumped by the roadside to make it look like a road accident.
"We don't believe it was an accident," Kritsada's father Suriya, 47, said as he and his wife met officers at the Crime Suppression Division (CSD) on Monday.
Last year the couple also travelled to Bangkok to petition the CSD for help. Their demand for a fresh probe comes less than a month after they claimed they had evidence to counter officers at Thawatchaburi police station who backed off from launching a murder investigation.
Mr Suriya said he wants to know what the CSD thinks after its investigators looked into the case and gathered new clues in the area. He said he came to meet them to ask how much progress had been made.
He believed accounts of witnesses and security camera footage may have shed light on the incident.
The video recordings show a person appearing to use a mat to wrap what the parents believe is their son's dead body before driving to leave it on the Roi Et-At Samat road, according to Ronnarong Kaeophet, president of the Network for Campaigning for Justice in Society, which gave legal advice to Kritsada's family.
Mr Suriya says he also suspects one witness who was a friend of Kritsada. The two met before the tragedy but, during interrogation, the boy told police he had never met Kritsada, Mr Suriya claims.
The boy also allegedly phoned other friends of Kritsada's and "threatened" them into not speaking about the death, Mr Suriya added.
"So how can I believe my son died in an accident?" he asked.
However, Pol Col Bunlue Phadungthin, chief of CSD Subdivision 3, said the CSD was unable to reach any conclusions.
After months of investigation, his team could not discount either Thawatchaburi police or the boy's parents' versions of events.
"The camera footage indicated by the family does not clearly suggest a murder plot and witnesses questioned by the CSD gave conflicting accounts," he said.
He said since the CSD decided to help investigate the case in order to try and provide some closure for the parents, its officers have been unable to find any concrete leads that could pave the way for a full-scale murder investigation.
Thawatchaburi police inspector Somsak Ketphibun insisted his officers dealt with the case in a fair manner and had carefully collected as much evidence as they could, questioning all witnesses and suspects.
A postmortem found a wound to Kritsada's head but concluded that it had not been the result of him being hit by a hard object.
"We did not rush to conclusions," Pol Lt Col Somsak told the Bangkok Post.
"We spent about a year investigating the case."
On June 14 this year, Thawatchaburi police reached a decision that the death had been accidental and forwarded the case to Roi Et prosecutors to conclude the matter.