The Justice Department (DOJ) has opened an antitrust inquiry into 4 major automakers who recently struck a deal with California to boost emissions standards for their nationwide fleets, The Wall Street Journal reports.

Why it matters: The report, if correct, signals the opening of a new and high-stakes front in the fight between California and the White House over vehicle emissions and mileage rules.

Where it stands: The WSJ reports that DOJ is seeking to determine if Ford, VW, Honda and BMW "violated federal competition law by agreeing with each other to follow tailpipe-emissions standards beyond those proposed by the Trump administration."

The story cites anonymous sources familiar with the matter. DOJ declined to comment.

The big picture: The reported probe comes as the Trump administration is preparing to freeze Obama-era carbon emissions and mileage standards, rather than allowing them to grow significantly stronger through the mid-2020s.

One big question: Whether the prospect of facing an antitrust probe could deter other automakers from joining the pact with California.

Quick take: The development deepens the complexity of the regulatory battle for automakers. The industry chafed at Obama-era rules, calling them too strict, but does not support President Trump's effort to freeze the standards outright.

The industry hopes to avoid having to make cars for a split U.S. market in which California, the largest U.S. market, and roughly a dozen states that follow its lead have one set of rules that are different than the federal standards.

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