The Department of Justice (DOJ) has issued civil subpoenas to four car manufacturers that agreed to a tailpipe emissions agreement with California over the summer, The Wall Street Journal reported Friday.

The Trump administration has been working for years to roll back tailpipe emissions standards, going head-to-head with California, which for decades has been permitted to enact tougher standards than the rest of the nation.

As negotiations between the state and administration hit a wall, California announced in July an emissions pact between Volkswagen, Honda, Ford, BMW and the California Air Resources Board.

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DOJ then reached out to the companies in late August to see if they coordinated with each other before agreeing to the state's guidelines. Such communications would raise antitrust questions, the department told the automakers.

Mary Nichols, chairwoman of the California Air Resources Board, has previously stated that the state worked individually with the automakers and that all parties were mindful of not violating antitrust laws.

The civil subpoenas, also known as civil investigative demands, indicate the initial talks between the DOJ and automakers did not fully resolve the DOJ's concerns.

A source close to the matter told the Journal that the subpoenas don't force the auto makers to turn over any documents, but instead ask them directly if they colluded before agreeing to the pact.

The DOJ's antitrust probe into the pact has prompted backlash among congressional Democrats, who say the investigation is just a means to further the administration's political agenda.

The move by DOJ follows an announcement from President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE that he would revoke the federal waiver California relies on to set tougher emissions standards. That announcement led to a lawsuit filed by 24 states.

Spokespeople for BMW and Honda told the newspaper that they were cooperating with the DOJ, while Ford and Volkswagen declined comment.

The four automakers did not immediately respond to requests for comment from The Hill.

Under the agreement reached by California and the automakers, the companies would reportedly meet annual emission improvement targets of 3.7 percent and defer to the state's authority to set its own emissions standards.

The Environmental Protection Agency, which oversees federal tailpipe standards, has argued that the country and automakers would be better served by one consistent standard.

The Hill has reached out to the White House, DOJ and the California Air Resources Board for comment.

Rebecca Beitsch contributed.