Charoen Wadaksorn, 37, was killed by two gunmen on a motorcycle Monday night in Prachuap Khiri Khan provincial town, south of Bangkok, after he alighted from the bus he had taken from the capital.

"We are investigating various motives behind the killings including extramarital love affairs, financial conflict, and his role as a leader of villagers in environmental campaigning," Lieutenant Wuttithep Pensaeng said.

Charoen travelled to Bangkok at the invitation of an upper house Senate sub-committee into corruption which is looking into a Bangkok businessman's bid to buy mangrove swamps in coastal Prachuap Khiri Khan.

He was also prominently involved in a successful campaign to force the government to scrap its proposal to build a coal-fired power plant near his village in Prachuap Khiri Khan.

Some 200 of Charoen's outraged relatives and supporters carried his body to Bangkok, arriving at the justice ministry where they demanded the country's most famous forensic scientist Pornthip Rojanasunan carry out the post mortem examination.

They also called for the Special Investigation Department -- Thailand's equivalent of the FBI which is under the authority of the justice ministry -- to handle the case instead of local police.

His death triggered immediate concern from the Senate Committee on Human Development and Security chairman Nirun Phitakwatchara, who called on the government to carry out a speedy investigation.

"Since 2001 he is the 16th local leader to be shot dead. The government cannot ignore this or consider it a minor point as they have sacrificed themselves for the public," Nirun said.

"The government should take measures to protect the rest," he added.

Nirun urged the government, and the justice ministry in particular, to hand the case over to the Special Investigation Department.

"We cannot trust the local police to handle this case as there are a lot of local influential persons involved," he said, using a common term for corrupt figures.

TERRA.WIRE