Wanted by the NIA in three cases, Basumatary was killed after he opened fire on security forces in Chirang district.
Police said he sneaked into the region a month back as a migrant worker. The shop, where he worked, had a few other migrant workers from Assam.
A 7.65 mm pistol, one rifle, two 7.65 mm live rounds and one grenade were recovered from him.
The home ministry had explained that AFSPA was only being extended to 20 km in all the bordering districts of Arunachal with Assam.
“We arrested them Friday and on Saturday produced them in court, which granted their transit remand till April 10,” an ATS official said.
Seven persons were arrested for allegedly selling pre-activated SIM card to NDFB-S militant.
Security forces arrested 12 top cadres, who included one central committee member, one deputy commandant and one platoon commander of the outfit.
The militants were apprehended in the joint operation by police, SSB and CRPF yesterday.
They were accused of massacre of the Adivasis at Pakriguri in Kokrajhar district on December 23.
At present, nearly 1.4 lakh people are staying at 68 relief camps in Kokrajhar.
The notification was issued after considering the law and order situation in Assam.
Assam government had recommended that the probe into these cases be handed over to the NIA.
Over 80 persons, many of them children and women, were killed in simultaneous attack in four villages.
Most of those taking shelter in Bengal’s camps are survivors of earlier violence.
The Assam government in a letter on Friday had recommended the investigation of these cases by the NIA.