The CBI sources said both arrived at the agency headquarters in the morning to appear before the investigation team.
Khaitan, who is an advocate and a former Board member of company Aeromatrix which was allegedly used to route bribe amount, is named in the CBI first information report as one of the accused.
The focus of the questioning is his alleged links with Italian middlemen -- Carlo Gerosa and Guido Haschkhe, CBI sources said.
The CBI had registered a case against Tyagi along with 13 others including his cousins and European middlemen.
The allegation against the former air chief was that he reduced flying ceiling of the helicopter from 6,000m to 4,500m (15,000ft) which put AgustaWestland helicopters in the race for the deal without which its choppers were not even qualified for submission of bids.
Tyagi has denied allegations against him and said that the change of specifications, which brought AgustaWestland into contention, was a collective decision in which senior officers of the Indian Air Force, the Special Protection Group, the National Security Advisor and other departments were involved.