Joshi, a suspect in the Samjhauta Express blast case, was shot dead in Dewas, Madhya Pradesh on December 29, 2007.
One Balveer was detained after interrogation from Indore's Shipra area, an NIA official told PTI from Delhi.
NIA teams carried out extensive searches at the residences of right-wing activists in Dakachiya and Manlauda villages, the official said.
Besides recovering eighteen items including swords, mobiles and letterheads, the NIA team also seized documents from a place near Indore, Shipra police station sources said.
Recently, the NIA had carried out extensive searches at the house of one of the accused, Tejram Pawar, in connection with the Mecca Masjid blasts in Hyderabad of 2007 in Depalpur, a suburb of Indore.
Searches may also continue tomorrow in and around Indore as well as Dewas districts, the official said.