Police and Defence sources said a group of seven to eight militants of National Democratic Front of Bodoland (NDFB) came to the couple's house at Shantipara-Sulekhaguri village at late Wednesday and tied their limbs before killing them with sharp weapons.
Before being killed, the couple was beaten up in front of their children who ran out to their neighbours' houses in fear, local people said.
The deceased man was identified as Buta Benga, an overground worker of the outfit, who was in possession of extortion money totalling Rs 45 Lakhs which belonged to NDFB-S dreaded leader Bidai, the sources said.
On December 22 last year in a joint operation by Army and police, Rs 5.48 lakhs were recovered and repeated attempts have been made to ascertain the whereabouts of the balance amount, they said.
Benga, who was in jail, was released on bail in the last week of March and was under constant threat from the group for the balance money.
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On this day two years ago, Bidai and his men had shot dead a school girl in front of the villagers in neighbouring Kokrajhar district charging her with being an informer for security forces.
NDFB-S had also gunned down 14 people and injured over 20 others at Balajan in Assam's Kokrajhar district on August 5.