The outfit informed a local village headman about the body lying in a dense forest 25 km away from Naginimara police station this morning, the sources said.
The headman informed the police which in turn informed its adjacent Bihubor police station in Assam, they said.
A joint team of Assam and Nagaland police recovered the body, which had two gun shot wounds on the head and sent it for post mortem at the Sibsagar Civil Hospital in Assam, sources said.
The insurgent outfit claimed Partha was a police informer and had also indulged in financial irregularities.
Asom (45) had joined the underground outfit in 1987 when he was a student of J N College in the district, they added.
The body of the slain ultra was taken to his home at Hatighuli, under the jurisdiction of Gaurisagar police station area in Sibsagar district, in a huge procession with the mourners shouting against ULFA-I 'commander in chief' Paresh Barua and saying "we don't need an Independent Assam".
