At least four people, including senior civil servants, will likely be summoned soon to face charges in connection with the murder of Karen rights activist Porlajee "Billy" Rakchongcharoen, a source on the investigation team probing the case said on Monday.
The charges include murder, robbery and illegal detention, according to the source.
The summonses are likely to be issued after the investigation team, led by the Department of Special Investigation (DSI), meets on Friday, said the source.
The investigation team believes it has sufficient evidence to seek arrest warrants for the four people.
"However, since some of the suspects are state officials, they will be called in to answer the charges," said the source.
Investigators are preparing the documents needed to press charges against the four, the source added.
"One of the four, who played a crucial role in the case, cannot be located at this time," the source said.
Authorities have solid evidence from months of investigation, which raised their chances of getting the suspects prosecuted on those charges, said the source.
Thanya Netithammakul, director-general of the Department of National Parks, Wildlife and Plant Conservation (DNP), meanwhile, said he has ordered the transfer of officials involved in detaining Porlajee prior to him going missing.
Porlajee was last seen on April 17, 2014, in the custody of park officials in Kaeng Krachan National Park.
It was officially a missing persons case until the DSI earlier this month ruled it to be a murder, following the discovery of several fragments of his skull.
The Public Sector Anti-Corruption Commission last week forwarded findings from its investigation into the case against the former chief of Kaeng Krachan National Park to the DSI.
The DSI recently linked the former park chief, Chaiwat Limlikit-aksorn, to Porlajee's murder.