Two inmates, Yusuf (21) and Abdus (22), killed one undertrial prisoner Thangminlien Zou of Churachandpur district by hitting him with blunt objects at about one AM in the jail, Additional DGP (Prison) P Doungel said.
As the death news of Thangminlien, imprisoned in connection with a murder case spread some inmates stormed into the prison cell and killed the two attackers, Doungel said.
The reason behind the attack was yet to be ascertained, the Additional DGP said.
Jail Superintendent Wungkhal Phanitphang said Yusuf and Abdus were nabbed in the border town of Moreh, neighbouring Myanmar in 2012.
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They had completed their prison term and were awaiting deportation, he said.
Three jail personnel, including jailor Th Brojen, were injured while trying to control the situation, the jail superintendent said.
They received minor injuries and were given first-aid by jail doctors, he said.
All the three bodies have been taken to Jawaharlal Nehru Institute of Medical Sciences (JNIMS).