The Justice Department has issued civil subpoenas to four auto makers that reached a tailpipe emissions deal this summer with the state of California, according to people familiar with the matter, the latest development in a federal antitrust investigation that has generated political controversy.

The subpoenas come on the heels of initial discussions between the department’s antitrust division and Ford Motor Co. F, -0.34%  , Honda Motor Co. HMC, +0.90%  , BMW AG BMW, +1.28%   and Volkswagen AG VOW, +2.50%   regarding a July agreement between the auto makers and the California Air Resources Board on fuel efficiency standards. The California framework is at odds with the Trump administration’s regulatory approach.

The Justice Department in late August began asking the companies whether they agreed among themselves on the outlines of the deal with California regulators.  If there was such collaboration, it could raise antitrust concerns, the department said.

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

The subpoenas, called civil investigative demands, are an indication that the initial dialogue between the auto makers and the Justice Department hasn’t fully resolved the matter. But the department’s formal demands also give the companies a mechanism to provide information that could clear them from suspicion.

An expanded version of this report appears on WSJ.com.

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