Former Air Force Chief SP Tyagi, who was arrested yesterday over alleged irregularities in the Rs. 3,600 crore deal with AgustaWestland to acquire a dozen VVIP choppers, has blamed the then Prime Minister office for tweaking the deal in favour of the UK-based helicopter maker. Mr Tyagi, his cousin Sanjeev Tyagi alias Julie Tyagi and a Delhi-based lawyer, Gautam Khaitan were sent to CBI custody for four days after being produced in a Delhi court today. Investigators have alleged that the men had allegedly received kickbacks. The first service chief, acting or retired to be arrested, Mr Tyagi, 71, was questioned for over four hours by the CBI on Friday.