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The U.S. Justice Department opened an antitrust inquiry into the decision by four major automakers earlier this year to voluntarily strike a deal with California to reduce automobile emissions, according to published reports.

In July, the automakers -- Ford (F - Get Report) , Volkswagen of America (VLKAF) , Honda (HMC - Get Report) , and BMW (BMWYY) -- said they reached an agreement in principle with California on emissions standards stricter than those being sought by the White House.

The Wall Street Journal first reported the news, citing people briefed on the matter. Reuters later reported that the antitrust division's chief, Makan Delrahim, sent Aug. 28 letters to the four automakers saying the government was concerned the agreement "may violate federal antitrust laws" but adding it had "reached no conclusions."

The Justice Department declined to comment on the reports.

BMW confirmed it had received a letter from the Justice Department. The company released a statement saying: "We look forward to responding to the Department of Justice to explain the planned (California Air Resources Board) framework agreement and its benefits to consumers and the environment."

The agreement reached between California and the four automakers, which account for about 30 percent of the United States auto market, allows for slightly lower fuel economy of 51 miles per gallon by 2026. California argues it has legal authority under the Clean Air Act to write air pollution rules that go beyond the federal government's.

In comparison, the Trump administration's plan would roll back those standards to about 37 miles per gallon. Automakers had struck the fleet-wide emissions deal with California hoping to head off years of legal battles and avoid a patchwork of state requirements.