The National Human Rights Commission will meet with every police team involved in Koh Tao murder case after a forensic unit determined the suspects might have been tortured during interrogation.
The police and officials from the CIFS give their testimony before the NHRC's committee on Monday.
Niran Pitakwatchara, chairman of the NHRC's sub-commission on civil and political rights, asked officials working on the case to testify on Monday. They were from the National Police and the Central Institute of Forensic Science (CIFS) under the Justice Ministry.
The NHRC was acting on two suspect-abuse complaints.
Pol Col Prachum Ruangthong, chief of Phangan police station, Khunying Porntip Rojanasunan, director of the CIFS, and the medical team who went to Koh Tao with the NHRC on Oct 13 was present at the hearing.
Dr Niran said that, after the murder, numerous police teams had been sent to Koh Tao as well as local police at the district, provincial, regional and central levels. Marine police, tourism police and crime suppression police were also there.
Each group had its own investigative team, Dr Niran quoted Col Prachum as saying. The officer said he could not interfere with the work of those units, said Dr Niran.
For Col Prachum's own unit, his direct supervisor sent Pol Maj Gen Suwat Chaengyodsuk from Bangkok police to collect all evidence and consolidate the investigations. However, Maj Gen Suwat came to the island after the two suspects had been detained.
The forensic team, accompanied by a translator, talked with suspects Zaw Lin and Win Zaw Htun on Oct 13 and found some interrogators might have tortured them, although it has yet to establish which team they belonged to.
The suspects told the doctors they had been beaten and suffocated with a plastic bag but the doctors did not find wounds or bruises on them. A physical check-up found a pressure mark on the chest of one of the suspects.
The NHRC will ask the jail's chief to have him x-rayed so they can find clues from orthopaedic pathology.
The two suspects have been detained since Oct 1.
"Seven to eight police teams worked at the same time so we need to ask all of them to give testimony. Col Prachum insisted it did not happen in his custody as there was a lawyer present during the interrogation," Dr Niran said.
Two Myanmar workers reportedly confessed to killing British tourists Hannah Witheridge and David Miller on the island on Oct 20. Witheridge, 23, and David Miller, 24, were bludgeoned to death near to where they were staying on the island on Sept 15.
Also on Monday, migrant-rights activists visited both Zaw Lin and Win Zaw Htun, getting to speak to them in Myanmar and English.
Activist Andy Hall of the Migrant Workers Rights Network tweeted following the visit that both said "their treatment was fine, they experienced no problems inside the prison but said were obviously unhappy in the prison."
Hall continued that both "seemed in good health, did not appear stressed or worried about anything," adding they were having some "difficulty with wearing chains on feet."
Both suspects also reported missing their families terribly, Hall tweeted. He said the two informed him that their parents were awaiting visas to travel to Thailand to see them.