The U.S. Justice Department is conducting an antitrust investigation of four automakers that have signed on with California in a deal to toughen tailpipe emissions standards.

Ford spokesman T.R. Reid confirmed Friday that the company received a letter from Justice informing it of the investigation. He says the company is cooperating in the inquiry.

In July, Ford, Honda, Volkswagen and BMW reached a deal with California to abide by standards that are tougher than those preferred by the Trump administration. The standards are closely linked with fuel economy requirements.

The move bypasses Trump's push to relax pollution and mileage standards nationwide that were set by the Obama administration.

The Justice Department wouldn't comment on the investigation.