BANGKOK (AP) — The parents of a self-exiled Thai activist who disappeared after reportedly being extradited from Vietnam visited government offices and diplomatic missions in Bangkok on Monday to seek information about his fate.

Siam Theerawut is one of three exiles about whom human rights groups have expressed concern about because neither Thai nor Vietnamese authorities acknowledge holding them.

Siam, Chucheep Chivasut and Kritsana Thapthai fled Thailand after a May 2014 military coup to neighboring Laos, where they continued political activities critical of their homeland. Thai authorities have been seeking their return on charges of insulting the monarchy, which is punishable by up to 15 years in prison.

There is increased fear for the safety of Thai dissidents after three others disappeared from Laos last year, and the eviscerated bodies of two of them were found later in the Mekong River.

Siam's parents, accompanied by sympathizers, presented letters Monday to Thailand's National Human Rights Commission, the Vietnamese Embassy and the European Union's office.

Theerawut's mother, Kanya Theerawut, told reporters that Thailand's Crime Suppression Division police said they have no information about his case.

"I want information from Vietnam's embassy on whether Siam was deported to Thailand or not. I'm most worried for his life," she said.

In a letter addressed to Vietnamese Ambassador Nguyen Hai Bang, she said she hoped the ambassador would look after the rights of the Thai people and not extradite refugees who risk harassment.

New York-based Human Rights Watch said the three were handed over to Thai authorities on May 8, though Thai officials have denied any knowledge of such a transfer.

"Only by publicly affirming that these three activists are in detention and in contact with their relatives and legal counsel will the authorities put to rest the fear that these men have been forcibly disappeared, " Brad Adams, Asia director at Human Rights Watch, said at the time.

Many of those who fled to Laos after the 2014 coup are associated with Thailand's anti-military Red Shirt movement, which staged aggressive street protests in Bangkok in 2010 that were violently crushed by the military.

Chucheep has been a particularly high-profile target as he has produced an online radio program in which he has been critical of both Thailand's military and its constitutional monarchy.

Siam, Chucheep and Kritsana are believed to have moved to Vietnam after their fellow Thai activists in Laos were killed. Their deaths raised concern among fellow activists that they were kidnapped by a death squad, either vigilante or officially sanctioned.

Last September, Thailand's military government linked activists in Laos to a tiny political movement advocating the abolishment of the constitutional monarchy. They arrested several of the group's sympathizers inside Thailand on charges of sedition, seizing as evidence T-shirts bearing the group's logo of a red and white flag.

Thailand's Deputy Prime Minister Prawit Wongsuwan, a former army chief and member of the ruling junta, commented at the time that those detained were part of a network of outlaws that had fled to Laos to evade charges of sedition and insulting the monarchy. He called them "traitors" and said they would all be arrested.

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This story has been corrected to show that a letter was presented to the European Union, not the European Commission.

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