(Reuters) -The U.S. Department of Justice has opened an in-depth antitrust investigation into Nippon Steel's ( NISTF ) $14.1 billion takeover of U.S. Steel, the Politico reported on Wednesday citing two people with direct knowledge of the matter.

The DOJ declined to comment, while the companies did not immediately respond to Reuters' requests for comment.

The deal has faced the scrutiny of U.S. lawmakers over national security concerns, with President Joe Biden saying last month U.S. Steel must "remain an American steel company that is domestically owned and operated".

During a visit to the U.S., Japanese Prime Minister Fumio Kishida said he hoped the deal would proceed in a positive direction but did not criticize the DoJ's scrutiny of the takeover.

The deal has also faced the disproval of the United Steel Workers union.