WASHINGTON—The Justice Department closed its antitrust investigation of four auto makers that had reached a tailpipe-emissions deal last summer with the state of California, finding no evidence of collusion among the companies.

After examining the matter for several months, the department concluded that the auto makers—Ford Motor Co., Honda Motor Co., BMW AG and Volkswagen AG—didn’t engage in conduct that violated U.S. antitrust law, according to people familiar with the matter.

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