The Department of Justice is investigating four automakers for signing a deal with California officials in July to strengthen tailpipe greenhouse gas emissions regulations.

Ford Motor Co., Honda Motor Co., BMW and Volkswagen in July signed an agreement with the California Air Resources Board to limit tailpipe emissions, but the federal government is now claiming that the Clean Air Act and other laws prohibit states from setting motor vehicle fuel-economy standards.

In July, Michael Abboud, a spokesman for the Environmental Protection Agency, said California's deal with automakers is a "PR stunt that does nothing to further the one national standard that will provide certainty and relief for American consumers."

But California Gov. Gavin Newsom said that clean-air emissions standards are "perhaps the most significant thing this state can do, and this nation can do, to advance [climate change] goals. The Trump administration is hellbent on rolling them back. They are in complete [denial] about climate change."