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The parents of a self-exiled Thai activist who disappeared after reportedly being extradited from Vietnam have visited government offices and diplomatic missions to seek information about his fate.

Siam Theerawut is one of three exiles that human rights groups are expressing concern about because neither Thai nor Vietnamese authorities acknowledge holding them.

All three 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.

After several fellow Thai activists in Laos were abducted and killed earlier this year, the three are believed to have moved to Vietnam.

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