CBI sources said they were looking into links among Tyagi, his brothers, Khaitan and company and middleman, through whom the bribe was routed to India.
The CBI is also looking into Tyagi's international visits after his retirement and his bank accounts and assets.
Khaitan, who joined the CBI investigation, claimed that during questioning he admitted to have setting IDS Tunisia.
According to sources, IDS Tunisia is a subsidiary of IDS India, one of the companies through which the alleged bribe was routed by AgustaWestland middlemen to other countries.
On Thursday, Khaitan will be questioned by the CBI, while Tyagi will join the Enforcement Directorate for questioning.
Meanwhile, the CBI on Wednesday conducted raids at the Gurgaon residence of Tyagi.
CBI sleuths also conducted searches in 12 places in Delhi, Gurgaon and Chandigarh in connection with the alleged scam.
Satish Bagrodia, the brother of former union minister Santosh Bagrodia, has also been named in the FIR.
The CBI team had registered a preliminary enquiry on the basis of some important documents that it had obtained from Italian probe agencies in connection with the alleged payment of kickbacks by AgustaWestland to bag the chopper deal. A complete case would be registered once sufficient evidence is procured.
The Defence Ministry had earlier threatened to cancel the chopper deal and sought a CBI probe after the CEO of AgustaWestland's parent company, Finmeccanica, Giuseppe Orsi, was arrested in Italy for his role in paying bribes.
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