JAKARTA (REUTERS) - Indonesia's biggest environmental group on Thursday (Oct 10) called for an independent investigation into the death of one of its activists after pointing out suspicious circumstances and saying his work could have made him a target.

Mr Golfrid Siregar, an activist with the Indonesian Forum for the Environment (Walhi), was discovered by a pedicab driver lying unconscious on the side of a road in the city of Medan on Sumatra island last Thursday, police said.

He was taken to hospital but died three days later, police said.

Police initially said it appeared to be a traffic accident, but on Thursday, Medan's chief detective, Mr Eko Hartanto, said three men, including the pedicab driver, had become suspects for stealing the victim's belongings, state news agency Antara reported.

"We have also done a reenactment of the crime scene to find what exactly caused his death," the news agency cited Mr Eko as saying.

Walhi said in a statement this week that Mr Siregar suffered injuries to his head that appeared to come from a blunt object, but there were no wounds on other parts of his body.

His bag, laptop, wallet and phone were missing, it said.

In a joint news conference in Jakarta on Thursday, Amnesty International Indonesia researcher Papang Hidayat said there should be an independent investigation "because the victim had raised cases on environmental and human rights crimes against local people, whose network of perpetrators don't only involve corporations but also the state apparatus".

Human rights lawyer Muhamad Isnur also questioned why Mr Siregar's motorbike had not been stolen if his death resulted from a robbery.

Police detective Hartanto and a North Sumatra police spokesman did not respond to requests for comment.

Mr Siregar had been involved in various projects in recent years, including helping fishermen in a lawsuit against a sand miner and assisting residents in a fight against forest encroachment, said Walhi's head of advocacy, Mr Zenzi Suhadi.

According to Mr Suhadi, his latest and most high-profile case was representing Walhi in its legal challenge to stop the construction of a US$1.5 billion (S$2.07 billion) China-funded hydro dam, which some experts had warned would destroy the habitat of orangutans.

North Sumatra Hydro Energy (NSHE), which owns the dam project, said by text message: "We are requesting all parties not to speculate and connect PT NSHE directly or indirectly to the death of Golfrid Siregar... until the results of a police investigation come out."