Updated

Thai authorities were urged Monday to investigate the disappearance of an environmental activist who has worked to help ethnic Karen villagers report on illegal activity around Thailand's largest national park.

Por Cha Lee Rakcharoen, known as "Billy," was detained briefly in the park last Thursday for carrying illegal wild honey but was released and has not been seen since. At the time he had been heading to meet villagers and activists in preparation for a lawsuit that would accuse park officials of burning and destroying the homes and property of more than 20 families in the area.

Kaengkrachan National Park, neighboring Myanmar in Petchaburi province, is mostly rainforest with a variety of bird and mammal species. It's had problems with illegal logging and wildlife poaching.

Park chief Chaiwat Limlikitaksorn said Billy was released because the honey he was allegedly carrying was deemed a petty offense.

A park intern, Issara Seuksahet, told The Associated Press by phone from Chaiwat's office that Billy was seen riding a yellow motorcycle in the rain after he was released.

Police Col. Woradet Suanklaai said his office was investigating but Billy's whereabouts remained unknown. Billy's wife and a group of villagers met Monday with the police and the Petchaburi provincial governor to call for them to solve the case.

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.

Human Rights Watch also expressed concern for Billy's disappearance and said the park chief, Chaiwat, was being investigated for allegedly masterminding the killing of an activist from Billy's activism network in 2011, who had helped ethnic Karen villagers report on abuses, violence, illegal logging, and poaching allegedly committed by park officials.

Chaiwat has not been suspended from duty as required under disciplinary regulations regarding officials under criminal investigation, Human Rights Watch said. "Chaiwat's presence at the national park has been a cause of fear among local activists and villagers, particularly those involved in lawsuits against him," it added.