A federal antitrust investigation in California surrounding auto emissions has accelerated.
The Justice Department has issued civil subpoenas to four auto makers that combined this past summer on a tailpipe emissions deal with the state of California, according to The Wall Street Journal.
The DOJ conducted discussions with Ford, Honda, BMW and Volkswagen concerning the July agreement between the auto makers and California over fuel efficiency standards.
California is at odds with the Trump administration over the standards.
BMW and Honda said they are cooperating with the department.
Spokesmen for Ford, Volkswagen and the Justice Department declined to comment to the Journal.
The department says if there was a collaboration between the companies on the standards, it could raise antitrust concerns.
Last week, other automakers including General Motors, Fiat Chrysler and Toyota intervened in a lawsuit to defend a Trump administration effort to revoke California’s ability to set its own emissions standards.
The legal fight could make it to the Supreme Court as an issue of states' rights.
News of the antitrust inquiry was reported by The Wall Street Journal in September.
President Trump has been trying to roll back the targets for reducing emissions first agreed to by the Obama administration and California in 2012.
The goal was for automakers to achieve roughly 5 percent annual increases in fleetwide fuel efficiency through mid-decade.
Under the framework of the deal, the goal would be for annual emission improvement targets of 3.7 percent and not challenge California’s authority to set its own standards.
The Journal reports the administration wants to set annual targets of 1.5 percent.