Surapong Kongchanthuk, a lawyer for Porlajee's wife, said at the DSI earlier today (Dec 23) that officials had charged Mr Chaiwat and three others with premeditated murder, unlawful detention, physical assault, robbery, body concealment and malfeasance.
The robbery count lists honey, a shotgun, motorcycle and camera.
"The DSI case has drawn global attention because Billy was a human rights activist who did not have an enemy. The decision to press the charge shows the international community that the Thai justice system meets international standards. It gives confidence to people working in the area of human rights," Mr Surapong said.
Pol Lt Col Shane Kanchanapach, chief of the DSI's Bureau of Regional Operations, said the 17 folders containing the investigators' case report would be delivered to the Office of the Attorney-General on Monday, and public prosecutors would have 40 days to handle the report.
"The DSI has done its best in gathering witnesses and evidence in the case," he said.
Pol Lt Col Shane said Mr Chaiwat and other accused failed to send in their written statements as they had said they would. The DSI had set Nov 28 as the deadline.
Pinnapa Prueksapan, Porlajee's wife, said she was satisfied with the DSI's casework. It showed that the justice system does work. She said the Justice Ministry was taking good care of her, and she had received no threats.
Porlajee, 31, disappeared in April 2014. He was helping his relatives sue Mr Chaiwat over the torching of their bamboo huts and rice barns during a series of forest evictions in Kaeng Krachan National Park in Phetchaburi province.
Porlajee was last seen alive on April 17, 2014, after Mr Chaiwat and officials under him arrested the Karen activist for allegedly collecting wild honey in the forest.
Mr Chaiwat and three others surrendered to the DSI last month. They denied all charges. The DSI sought their detention in custody, but the Central Criminal Court for Corruption and Misconduct Cases released them on bail of 800,000 baht each.
The DSI earlier said it had found convincing evidence in the national park in Phetchaburi province, and believed Porlajee had been murdered and dumped in the Kaeng Krachan dam reservoir.