CBI sources said Managing Director of Vectra group Anil Mansharamani and a city-based businessman were also part of questioning.
The agency sources claimed they had got enough prima facie evidence of Singh's relations with arms dealers, including Rishi, which could have prompted him to make an offer to the then Army Chief.
Singh has denied having any links with Rishi and even said no such offer was ever made by him.
"Investigation will reveal that I am not involved and also the reasons for someone raking it up one and half years after the alleged incident," he had said in reply to an SMS sent for his reaction.
The then Army Chief Gen (Retd) V K Singh had alleged that Singh, who retired as chief of Defence Intelligence Agency in July 2010, offered him a bribe of Rs 14 crore in his office in September, 2010 to clear the purchase of a tranche of 1,676 Tatra BEML trucks, a matter he had reported to Defence Minister A K Antony.
Rishi is a Director in the Tatra Sipox UK which supplied trucks through public sector undertaking BEML.
He is also an accused in CBI FIR related to the truck supply deal.
CBI had initiated a preliminary enquiry in April after getting a formal complaint from the then army chief.
Singh had refuted the allegations and also slapped a defamation case against Gen Singh.
The agency, after six months of preliminary investigation, had converted it into a regular case of alleged violation of Prevention of Corruption Act as it found enough prima facie evidence against Tejinder Singh.
