Fears are growing over the fate of three Thai anti-monarchy fugitives said to have been turned over by Hanoi to Bangkok, although the Thai government denies any knowledge of their whereabouts.
Their disappearance comes just five months after the badly mutilated bodies of two other anti-monarchy activists were found in the Mekong River.
Political activists Chucheep Chiwasut, Siam Theerawut and Kritsana Thapthai were detained by the Vietnamese authorities early this year and sent back to Thailand last Wednesday, according to an activist group - Thai Alliance for Human Rights.
But the Thai authorities, including Deputy Prime Minister Prawit Wongsuwan, say they do not know of any repatriation.
Reached yesterday, Thailand's Foreign Ministry spokesman Busadee Santipitaks said: "I have seen the news reports and have no further information."
Police Lieutenant-General Sarawut Kanphanit, commander of the special branch unit, described the attempts to link the kingdom to the three missing fugitives as a rumour that is being used to discredit Thailand as a brutal country.
He told The Straits Times: "This is just a rumour. No reliable source can be identified from both countries. We haven't had any report of these people and there is no report of their arrest in Vietnam or any other country."
Mr Siam's mother Kanya Theerawut lodged a petition with the Vietnamese embassy yesterday asking for information about her son.
"I am worried for his life," she said, addressing reporters in front of the embassy in Bangkok. "Our country has laws. I want them to follow the law."
Three political activists who broadcast anti-monarchy and anti-junta messages from their hideout in Laos had similarly gone missing on Dec 12 last year.
Later that month, two bodies were found in the Mekong River that forms the border between Laos and Thailand.
Their "hands and feet were bound and their faces smashed beyond recognition", said Human Rights Watch.
DNA tests confirmed that the bodies were those of aides of prominent anti-monarchist Surachai Danwattananusorn, who fled Thailand in 2014 shortly before the current military government was installed by a coup. Mr Surachai remains missing and is thought to be dead.
Thailand is a constitutional monarchy where insulting or defaming the king, queen, regent or heir apparent is punishable by up to 15 years in prison on each count.
But there have been no fresh lese majeste prosecutions since last year, something attributed to the intervention of newly crowned King Maha Vajiralongkorn.
Still, the Thai government has accused Mr Chucheep - whose nickname is Uncle Sanamluang - of heading a secessionist movement from Laos. Mr Chucheep and his two aides moved from Laos to Vietnam shortly after the murder of their colleagues.
Mr Sunai Phasuk, senior researcher for Human Rights Watch, told The Straits Times: "The reaction of both the Thai and Vietnamese authorities gives us no confidence at all about the safety of the three missing activists."
Activists have also speculated about a quid pro quo between Vietnam and Thailand on the extradition of their respective dissidents.
In January, Vietnamese journalist Truong Duy Nhat disappeared in Bangkok, after applying for asylum at the United Nations High Commissioner for Refugees there. In March, he was found in a Hanoi jail.
"These are serious allegations... they need to have serious investigation and clear explanation," said Mr Sunai.