Well-known activist Sutthi Atchasai was found dead with a gunshot wound to his temple inside his car parked at his home in Rayong province yesterday.
Police have not ruled out suicide, but are also looking at other causes.
Police have already spoken to his mother, who initially agreed that it was a suicide, though they need to speak to his brother too, who is currently busy arranging the funeral.
Senior officer Pol Colonel Kiti Sirikhot said there were three bullet holes in the pickup truck's windscreen, with apparent signs of the bullets being shot from inside the vehicle. A bullet was also found inside the car, with the pistol in the late activist's hand and gun residue on his palm. There were no signs of struggle inside the car though the rubberised part of a wiper blade was found at the scene, he said.
A man close to Sutthi told the press that just two days ago Sutthi was complaining about not being able to take up his role as an activist and allegations questioning his reduced role. Also, Sutthi was allegedly in debt after his earlier restaurant business folded and he ended up having to mortgage his mother's land to absorb the loss.
The source also quoted Sutthi as saying that he was being "lobbied" to give up his role as activist for an extended period and that he had taken out loans to cover the expenses of previous rallies that were mostly about environment issues in the East and Rayong.
"Even after he became heavily indebted, Sutthi was still subject to allegations," the source said.
The late activist's funeral will be held at Wat Tree Mit Pradittharam in Muang district, though the cremation has yet to be scheduled.
Sutthi's brother Mongkhol said later that his late sibling had complained about the lack of money due to the losses incurred by the restaurant, which was closed a year ago. He even asked to borrow from Mongkhol.
The late 38-year-old Rayong native was instrumental in campaigning against local industrial projects that were found to be violating environmental laws. A graduate from Ramkhamhaeng University, Sutthi took up social activism as a coordinator of slum communities in 1994, and related charities later, before becoming the coordinator of Chumchon Thai, a group helping the urban poor, in 1999.
He was also a member of a prominent four-party committee, chaired by former prime minister Anand Panyarachun, that was part of the legal squabbling against a large-scale toxic control management and impact assessment associated with the Map Ta Phut a few years ago.
Sutthi set up the Network of the Eastern People in 2005 to campaign for people's basic civil rights and against industrial projects that do not follow toxic safety controls and environmentally related regulations.