The U.S. Justice Department is reportedly launching a preliminary investigation into several automakers for reaching agreements with the state of California regarding emissions standards. Ford Motor Company, Honda Motor Company, BMW AG and Volkswagen AG all reached agreements with California to achieve higher emissions standards than those set by the Trump Administration.

Reuters reports the antitrust division of the Justice Department has sent letters to each of the four automakers confirming it had opened preliminary investigations into agreements made with California. The agreements effectively sidestep the Trump Administration’s rollback of Obama era emission standards, which has made all four targets of President Trump.

Thus far the Justice Department has declined to comment on the matter. Ford and Honda have confirmed their receipt of the letter, but have declined to provide any additional context on the matter.

On Friday, general counsels at the Environmental Protection Agency told California Air Resource Board Chair Mary Nichols in a letter that its actions in connection with the voluntary agreement “appear to be unlawful and invalid.”

On Thursday the Trump Administration announced plans to proceed with stripping California of its waiver under the Clean Air Act to set its own vehicle emissions rules. California sought a waiver after the Trump Administration shuttered Obama era emission standards that called for a fleet-wide average of 46.7 mpg by 2026. Trump’s rules call for the average to be 37 mpg during the same period.

Trump’s emission plan actually raises U.S. oil consumption by 500,000 barrels per day in the 2030’s, but reduces costs on automakers by an estimated $300 billion over the period.