Boeing Co. executives told officials on President-elect Donald Trump’s transition team that the cost of a new version of Air Force One could be lowered if the government agrees to reduce its requirements for the plane, people familiar with the discussions said.

“Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion,” Trump said in his tweet.

The Air Force and Boeing are still conducting work to reduce the program’s technical risks before the company is awarded an advanced development contract, Captain Michael Hertzog, a spokesman for the service branch, said in an e-mail.

The Boeing executives who contacted Trump officials suggested that the price of the new planes could be reduced if the Air Force and Secret Service revise their specifications for the aircraft, the people familiar with the discussions said.

T’s no clear alternative to Chicago-based Boeing to build a new Air Force One, especially not for a president who wants to buy U.S. products.

The Air Force said last year that it was giving Boeing the contract without competition because its plane is the only one manufactured in the U.S. “That when fully missionized meets the necessary critically important capabilities” that a president needs.

Trump previously held stock in Boeing but sold all his equity holdings in companies in June, Miller said.