In its ongoing investigation in the Rs.3,767 crore VVIP chopper scam, the CBI is focusing on the roles of several top political leaders, sources revealed on Thursday.

The CBI sources said that the leaders include former Prime Minister Manmohan Singh, Congress President Sonia Gandhi and her aide Ahmed Patel.

They came under the agency's radar following the arrests of former Indian Air Force chief S.P. Tyagi, his cousin Sanjeev Tyagi and Delhi-based lawyer Gautam Khaitan. The trio was now out on bail.

The agency was also looking into the role of the then Defence Minister Pranab Mukherjee.

Sources revealed that the probe would cover all the leaders in the erstwhile Congress-led-United Progressive Alliance (UPA) regime, who were alleged to be connected when the deal for the 12 VVIP choppers was being finalised.

The agency would be taking action on the basis of two notes by Italian middleman Christian Michel that were taken into cognizance by an Italian court.

The court had earlier in April 2016convicted former Finmeccanica chief Bruno Spagnolini and Giuseppe Orsi, who headed the chopper division AgustaWestland, for bribing Indian officials.