The chargesheet was filed before the Special NIA Court in Guwahati on Friday.
Gulzar Hussain, an ADSP in Sonitpur, and a civilian, Anlark Basumatary, were killed during the ambush, and many police personnel were injured.
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The NIA took over the probe in the case in August 2019 and concluded that members of the separatist outfit had laid the ambush.
Narzary and Nitul Daimary have been convicted in another case, in which the former has been given a death sentence, while the latter has been sentenced to life imprisonment.
Kham Kham Daimary and Junu Basumatary – two other cadres who were involved in the case -- were killed in separate encounters with security forces.
Further investigation in the case continues.