During an in-camera proceeding, the agency told this to District Judge I S Mehta who today extended till September 25 the judicial custody of Akhtar, Zia-ur-Rahman and alleged IM operative Faizan Ahmed Sultan in connection with the case.
According to sources, the NIA told the court that their probe into the case against Akhtar and Pakistani national Zia-ur-Rahman was over and they would file a supplementary charge sheet against them within this week.
Sultan was arrested by the NIA on May 2 at the Indira Gandhi International Airport here after he was deported from Sharjah.
According to the NIA, Sultan was wanted in various cases here for allegedly conspiring to wage a war against the country, raising funds for terrorist acts and recruitment of persons for the banned outfit.
The special court had earlier allowed the NIA's plea seeking extension of time by three more months to complete its probe against Tehsin and Zia-Ur-Rahman in the case.
The agency had filed two charge sheets in the case against nine suspected IM operatives, including its co-founder Yasin Bhatkal and his close aide Asadullah Akhtar alias Haddi, who were arrested by the NIA on August 28, 2013 in the case relating to a larger conspiracy of IM to carry out terror activities across the country.