The apex court, however, fixed his jail term of 20 years without remission, over the period already undergone.
"While altering the death sentence to that of imprisonment for life, we are inclined to fix the term of imprisonment as 20 years without remission, over and above the period of sentence already undergone, which, in our view, would meet the ends of justice," a bench of justices K S Radhakrishnan and Vikramajit Sen said.
"Viewed in that perspective, we are of the view that this is not a rarest of rare case for awarding death sentence," it said.
The court passed the order on an appeal filed by convict Ashok Debbarma challenging Agartala bench of Gauhati High Court upholding death sentence awarded by trial court.
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Later the CID filed a charge sheet against 16 insurgents, but 11 of them were absconding.
Additional and Session Judge of West Tripura district released three of the rest five insurgents unconditionally as no evidence was found against them.
Meanwhile, the two accused Ashok Debbarma and Gandhi Debbarma managed to flee from police custody before the verdict. The police later arrested Ashok, who was sentenced to death.