BANGKOK (AFP) - From the moment he was dragged into a car in broad daylight on the streets of Phnom Penh on June 4 last year, nothing has been seen or heard of Thai activist Wanchalearm Satsakit.
Thai and Cambodian police insist they are investigating, but with the probe into his apparent abduction going nowhere, his sister Sitanun has vowed to pursue the truth "to the last".
Rights campaigners say the case sends a chilling signal of Thailand's willingness to pursue political dissidents beyond its borders.
Mr Wanchalearm, aged 37 at the time of his disappearance, was wanted by Thai police for running an acerbic anti-government Facebook page and for posting satirical videos criticising Prime Minister Prayut Chan-O-Cha.
Ms Sitanun's frustration at the lack of help she has had from the Thai authorities has not dimmed her determination to find out what happened to her brother.
"If Wanchalearm's case was like other cases, where relatives of the victims did not come out to demand (justice) or pursue the case, I think nothing would materialise," she said. "But in my case, I will not let this go."
In December she travelled to Cambodia to present evidence to a Phnom Penh court and press for a serious investigation.
Mr Wanchalearm was living in self-imposed exile in Phnom Penh, having fled Thailand in the wake of the 2014 coup led by Mr Prayut, the army chief at the time and now prime minister.
He was wanted for sedition and for breaching Thailand's Computer Crimes Act, broad legislation which rights groups say is abused to silence critics.
Cambodian police say they have made no progress in the case and - having refused initially to even launch an investigation - still cast doubt on the idea Mr Wanchalearm was abducted.
"We don't have any new information so far. We are trying our best to collect any clues to confirm whether or not such a case happened in Cambodia," Mr Chhay Kim Khoeun, Cambodian National Police spokesman, told AFP.
And he refused to set a timeframe for the investigation to be completed.
Mr Korrawat Panprapakorn, director-general of Thailand's Department of Special Investigation, which handles complex and sensitive cases, told local media his team was investigating.
But he said that the onus was on the Cambodian authorities to lead the probe, as the incident took place on their territory.
Ms Sitanun was speaking to Mr Wanchalearm on the phone as he was bundled into a black SUV by armed assailants on June 4 last year.
She heard him shouting "I can't breathe" and muffled noises before the line was cut off 30 agonising minutes later.
His disappearance became a rallying point for a youth-led pro-democracy movement in Thailand, which at its peak drew tens of thousands to the streets last year.
The movement demanded Mr Prayut's resignation and changes to the kingdom's military-drafted constitution.
Human Rights Watch says the Thai and Cambodian authorities have been reluctant to investigate a "hideous crime that happened in broad daylight".
"The absence of justice for exiled Thai dissidents sends a spine-chilling message that no one is safe and they can end up like Wanchalearm," senior researcher Sunai Phasuk told AFP.
Two bodies with concrete blocks stuffed in their stomachs were found on the Thai side of the Mekong River in late December 2018.
The pair were identified after DNA tests as Mr Kraidej Luelert and Mr Chatchan Buphawan, Thai political dissidents living in Laos.
At least nine pro-democracy activists living in exile have disappeared in recent years, according to Human Rights Watch.
Even before he disappeared, Mr Wanchalearm's girlfriend Prakaidao Pruksakasemsuk recalled, the couple felt they were being watched when she visited him in Cambodia.
It started when he picked her up at the airport and she said they were frequently trailed including at their favourite night haunt.
"They asked bar staff where we lived. The staff took photos of them and showed them to us," Ms Prakaidao told AFP, adding that the watchers all had the crewcut hairstyle favoured by Thai police and military personnel.
Ms Prakaido, still deeply traumatised by Mr Wanchalearm's disappearance, says she is "disappointed in every way" with efforts to find out what happened to him.
"This is not far-fetched. It could happen to you, your children who attend pro-democracy rallies or friends and relatives," she said.
On Wednesday, ahead of the anniversary of the disappearance, Ms Sitanun and a legal team submitted a formal complaint to the Thai authorities.
"I still have a duty to continue searching and pressing this matter to the last," she said.