The Trump administration says a deal between California and four carmakers to improve fuel efficiency may be illegal. The Justice Department has also launched a probe to see whether it violates antitrust laws. Together, the moves raise the stakes in a months-long standoff over efforts to weaken a key Obama-era climate rule.

The administration is moving to roll back fuel economy standards despite resistance from some in the industry. In July, BMW, Ford, Honda and Volkswagen reached a deal with the state to keep improving fuel efficiency even if federal standards are weakened. The antitrust probe seeks to challenge that deal. T.R. Reid, a spokesperson for Ford, said in a statement that the company “will cooperate with respect to any inquiry.”

Also at stake is a decades-old waiver that lets California set its own, stricter emissions standards, which a dozen other states also follow. On Friday, the EPA and Department of Transportation sent the California Air Resources Board a two-page notice warning that its deal with the four automakers “appears to be inconsistent with Federal law.” It said that under the Clean Air Act, only the federal government has authority to set tailpipe pollution standards.

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