The medical team that performed the autopsy confirmed Sutthi fatally shot himself and was not assaulted by anyone, CIFS deputy chief Sakkaya Choomai told a news conference.
The 38-year-old activist was found with a gunshot wound July 15 in his pickup truck in his home garage in Rayong's Muang district. A .38 revolver was found in his right hand. He was rushed to Rayong Hospital and pronounced dead the next morning.
According to police, four shots were fired. The first three passed through the vehicle's windscreen and through the garage roof. The fourth bullet hit Sutthi in the right temple. All the shots were fired from inside the vehicle.
The examination indicated the fourth bullet was fired at a range very close to his skin and damaged the right brain, which caused paralysis on the left side of the body. This matched treatment results reported by the hospital. The killing bullet exited the frontal lobe on the left side of Sutthi's head and buried itself in the truck's roof, Mr Sakkaya said.
The CIFS's ballistics findings also matched that of police investigators at Muang Rayong police station, Mr Sakkaya added.
Gunshot residue was found on Sutthi's right temple but not on his right hand because tests must be carried out on the day of the shooting and Sutthi had been hospitalised before succumbing to his injuries.
On the suspicions why Sutthi, who is left-handed, shot the gun right-handed, a check discovered Sutthi actually was ambidextrous.
Sutthi's family doubted his alleged suicide and asked the National Human Rights Commission to investigate his death. The NHRC then asked the CIFS to examine the evidence and carry out the autopsy to conclude the cause of the death.
It is still not known why Sutthi fired three shots at his truck's windscreen before killing himself.
Police investigators said Sutthi’s mother and his nephew were at home before the suicide occurred. The nephew witnessed the incident and rushed to tell the woman that Sutthi had shot himself. Paramedics, who took the activist to hospital, also said the gun was in his hand, and his index finger was still on the trigger.
Sutthi led many environmental protests in his province, most famously against the construction of a power plant.
He had disappeared from the activist scene in recent years. Former senator Somchai Sawaengkarn, who worked with Sutthi on a number of campaigns, said last week he had been told the activist had accumulated work-related debts and mortgaged his family’s land.