The Justice Department has opened an antitrust inquiry into the four major automakers that struck a deal with California this year to reduce automobile emissions, according to people familiar with the matter, escalating a standoff over one of the president’s most significant rollbacks of climate regulations.

In July, four automakers — Ford Motor Company, Volkswagen of America, Honda and BMW — announced that they had reached an agreement in principle with California on emissions standards stricter than those being sought by the White House. The announcement came as an embarrassment for the Trump administration, which assailed the move as a “P.R. stunt.”

Now, the Justice Department is investigating whether the four automakers violated federal antitrust laws by reaching a deal with California, on the grounds that the agreement could potentially limit consumer choice, those people said. The Justice Department declined to comment on the investigation, which was reported by The Wall Street Journal.

The investigation comes amid a battle over the Trump administration’s effort to drastically roll back Obama-era rules intended to reduce emissions from cars and light trucks that contribute to global warming, a rollback that major automakers have publicly opposed. The administration is also considering a plan to revoke California’s legal authority to enforce stricter greenhouse gas emissions rules within its state borders, putting the two sides on a collision course.