
Saneh Yiamsawat died after being stabbed with a knife during a fight with a rival group of inmates at Klong Prem Prison in Nonthaburi province.
Another inmate, Prachuab Chaichanthuek, was seriously wounded in the incident, said the officials.
Four other prisoners involved in the brawl were held pending an investigation Sunday.
Prison commander Somsak Rangsiyopas said he had ordered a probe into the incident to find out if the attack on Saneh was masterminded by other parties that may have been involved in the murder of Khomkhai's mother, to silence him.
She was killed in a car bomb explosion in Phetchaburi province several years ago.
The Nation