The Justice Department is investigating whether the decision of four automakers in July to reach a voluntary agreement with California to adopt state emissions standards violated antitrust law, people briefed on the matter said. The antitrust division has sent a letter to the automakers indicating the Justice Department had opened a preliminary inquiry, the people said, and comes as the Trump administration has ramped up its opposition to automakers seeking to sidestep it on rolling back Obama-era fuel efficiency rules.
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