The chargesheeted CPI(Maoist) militants are Mohan Viswakarma of Imamganj, Bihar and Arjun Ganjhu of Jharkhand's Latehar district.
STF, Kolkata, had arrested Viswakarma from the city on July 25, 2012 and on the basis of the information provided by him, Arjun Ganjhu was arrested from Ranchi in Jharkhand the next day.
A large quantity of unfinished parts of about 800 hand grenades were seized from a transport agency and several workshops in the city.
"In view of the gravity of the offence, the Ministry of Home Affairs had directed the NIA to take up the further investigation of the case in April, 2013.
"The NIA, in its supplementary chargesheet, has charged the two accused persons for offences under the Unlawful Activities Prevention Act, for conspiracy to commit terrorist act, for being associated with a terrorist organization and for being a member of the same," an official release said.