His wife said he was murdered because of his opposition to an Irrigation Department dam project.

The wife of Mai Noinarai, Boonchu, told police her husband was killed over his activities against the Tha Sae dam.

But provincial police chief General Wisarn Plengkha remained unconvinced.

He said the dam was not a government priority and investigators suspected a personal disagreement or revenge was the motive.

Wisarn said police had a suspect but not enough evidence for an arrest. He said the killing was "not a professional hit". The murder weapon is a shotgun, widely available, he added.

Wildlife Fund Thailand deputy secretary-general Harnnarong Yaowalert encouraged the police to step up their search for the killer or killers irrespective of motive.

The fund works with villagers who have opposed the dam for the past decade.

"It is difficult say why Mai was killed unless we find the killers," Harnnarong said. "He was involved in the anti-dam movement now and then, and he was also active in local politics" and was instrumental in having a teacher removed for abusing a student. The teacher's relatives want him reinstated.

"But whatever the motive for the killing, it is the job of the police to find and arrest the culprit," he said.

Former Democrat member of Parliament for Chumphon Suwaroj Phalang is watching developments and is satisfied progress was being made.

"Mai was known for his determination to protect natural resources and many conservationists I talked to are saddened by his death. They admired him for his good work," Suwaroj said.