The Central Bureau of Investigation (CBI) on Friday summoned Former Air Chief SP Tyagi's brothers in connection with the Rs 3,600 crore VVIP helicopter deal.
Meanwhile, the investigative agency has also sought presence of lawyer Gautam Khaitan as it was not satisfied with his answer on receiving money from the middleman.
Tyagi and Khaitan were grilled for over 10 hours by the Central Bureau of Investigation on Wednesday.
Tyagi admitted to have financial interest in three companies - Banshi, Anuras and Shavan.
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.
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.