The Environmental Protection Agency (EPA) and the Department of Transportation (DOT) sent a letter to the California Air Resources Board, noting that their agreement with four automakers to have them adhere to stricter national emissions standards may violate the law.

The California Air Resources Board (CARB), led by chairman Mary Nichols, struck an accord in July with Ford, Volkswagon, Honda, and BMW to stricter greenhouse gas emissions (GHG) standards to light-duty cars and trucks and agreed to build vehicles to meet stricter than national emissions standards beginning in 2022.

California styled this agreement as “an alternative path forward for clean vehicle standards nationwide.”

However, the EPA and DOT wrote in a letter, obtained by Breitbart News, that the agreement might violate federal law.

The two federal agencies wrote, “Congress has squarely vested the authority to set fuel economy standards for new motor vehicles, and nationwide standards for GHG vehicle emissions, with the Federal government, not with California or any other State.”

The EPA and DOT’s letter to CARB arises as the Donald Trump Department of Justice (DOJ) launched an antitrust investigation into the four automaker’s agreement with CARB. The DOJ hopes to determine whether the companies have violated federal competition law by agreeing with each other to follow these GHG standards beyond those proposed by the Trump administration.

Ford, BMW, and Volkswagen did not respond to request for comment from the Wall Street Journal; however, Honda released a statement, saying, “Honda 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.”

While the EPA and DOT have warned against the CARB’s agreement with the automakers, the Trump administration has worked to roll back the Obama-era gas mileage standards. Studies have shown that the new Trump rule could reduce the price of an automobile by $2,500. Dozens of conservative leaders have cheered the Trump administration’s rollback of the Obama-era standards.

They noted that Section 209 of the Clean Air Act prohibits “California and other States” from adopting or attempting to enforce “their own emissions standards.” Further, the Energy Policy and Conservation Act (EPCA) “expressly preempts States from setting fuel economy standards for motor vehicles or taking any other action ‘related to’ the regulation of fuel economy.”

“Under EPCA and the Clean Air Act, it is DOT and EPA that have controlling authority to establish fuel economy and nationwide GHG emissions standards for new motor vehicles in the United States,” the two agencies added.

The DOT and EPA charged that “CARB’s actions in furtherance of the framework appear to be unlawful and invalid.”

The DOT and EPA urged Nichols to end the agreement between CARB and the four automakers, which may “result in legal consequences given the limits placed in Federal law on California’s authority.”