Four automakers — Ford (F) - Get Report Honda (HMC) - Get Report  BMW (BMWYY) and Volkswagen (VWAPY)  - are the subject of a U.S. Justice Department antitrust investigation over an independent agreement with California on vehicle-emissions standards, The Wall Street Journal reported on Friday.

People familiar with the situation told the Journal that department lawyers want to determine whether the automakers violated federal competition law by "agreeing with each other to follow tailpipe-emissions standards beyond those proposed by the Trump administration."

The new antitrust inquiry could increase tensions between Washington, Sacramento and the auto industry over Trump administration plans to roll back clean-air mandates on automakers, the Journal said. California is legally allowed to set emissions standards higher than those the federal government has set.

The Justice Department's antitrust division is "acting on its own accord and without direction from or coordination with the White House," one source familiar with the investigation told the paper.

The Journal said the automakers were sent letters last month requesting information and were told the department had not reached any conclusions. 

BMW and Volkswagen didn't immediately respond to the Journal's requests for comment.

Honda said it "will work cooperatively with the Department of Justice with regard to the recent emissions agreement reached between the State of California and various automotive manufacturers, including Honda."

Ford confirmed in an statement emailed to CNBC that it's been contacted by the DoJ on the matter. The automaker said it would "cooperate with respect to any inquiry." The DoJ declined to comment.