Por Cha Lee Rakcharoen, known as "Billy," was detained briefly in Kaengkrachan National Park last Thursday for possessing illegal wild honey but was released and has not been seen since.
At the time he was heading to meet villagers and activists to prepare for a lawsuit accusing park officials of burning and destroying the homes and property of more than 20 families in the area.
There are serious concerns about Billy's fate, partly because Thailand has a record of deadly violence directed at land and environmental activists, with perpetrators rarely brought to justice.
Billy's wife and a group of villagers met Monday with police and the provincial governor to urge them to solve the case.
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National Human Rights Commissioner Niran Pitakwachara also called on authorities to find the activist.
"Billy is not an ordinary villager who simply went missing. He is a key Karen activist who is fighting in a case in the Administrative Court, and it's the job of the government and the administrative officials to find out where he might be, whether he is being tortured or even killed," Niran said.