BANGKOK (Reuters) - A skull fragment found in a scorched oil drum dumped in a reservoir in Thailand belongs to a prominent ethnic minority rights activist who went missing in 2014, Thailand’s Department of Special Investigation (DSI) said on Tuesday.

Pholachi “Billy” Rakchongcharoen, an ethnic Karen land rights activist, was last seen on April 17, 2014, when he was detained by national park authorities at the Kaeng Krachan National Park in Petchaburi province, south of the capital, Bangkok.

Two pieces of bone fragment with burn marks were found in a 200-litre oil drum, submerged near Kaeng Krachan dam in May as authorities searched for evidence after years of fruitless investigation.

DNA taken from Pholachi’s mother showed the bone fragments came from her son, Korawat Panpraphakorn, deputy director of the DSI told a news conference.

“We found a piece of human bone which is part of a skull and based on a DNA test, it matches that of Billy’s mother,” Korawat said.

Twenty more bone fragments, which have not been tested, have since been discovered nearby, he said.

Korawat said that Pholachi was likely killed after he was abducted but the cause of death was not known. He declined to comment on any suspects, citing the investigation.

At the time of his disappearance, Pholachi was working with Karen communities on legal complaints against national park officials for the destruction and burning of houses and farms of families living in the park in a series of forest evictions.

The Karen are an ethnic minority many of whom live in communities in forests of northern and western Thailand, and over the b0rder in neighboring Myanmar. Many Karen in Thailand are stateless.

Pholachi was detained by national park officials in 2014 for alleged illegal possession of a honey comb from wild bees.

Park officers say he was released after questioning but Pholachi’s family said he has disappeared without trace.

Pholachi’s disappearance is one of more than 80 enforced disappearance in Thailand since 1980, according to rights group.