Mohan was asked to provide some documents regarding the procurement of components from Tatra Sipox UK owned by Rishi, the sources said.
The CBI has registered a case naming Rishi and unnamed officials of the defence ministry, army and BEML on March 30 for alleged criminal conspiracy, cheating and also under relevant sections of the Prevention of Corruption Act.
Both Natrajan and Rishi have denied allegations leveled against them.
The CBI is probing alleged irregularities in assigning of supply from then Czechoslovakia-based Tatra, with which the agreement was originally signed in 1986, to the Tatra-Sipox UK owned by Rishi in 1997 showing it as original equipment manufacturer and the fully-owned subsidiary of Czech company which was against the provisions of the defence procurement procedure, the sources said.
A controversy had come to fore after Army Chief V K Singh has said that he was offered a bribe of Rs 14 crore by a former Lt General to clear the deal of 600 trucks.
FAQ: All you wanted to know about the army-Tatra scam
Revealed: The real story behind CBI's Tatra probe
Tatra truck scam second BIGGEST after Bofors: BJP
Why India's defence preparedness is hollow
Army's flailing health is no BIG secret; THEY always knew