The Justice Department’s decision could boost the efforts of the auto companies and California to move ahead with tighter vehicle pollution standards than those being finalized by the federal government.

California’s governor, Gavin Newsom, welcomed the news. “These trumped-up charges were always a sham — a blatant attempt by the Trump administration to prevent more automakers from joining California and agreeing to stronger emissions standards,” he said in a statement. He called the decision “a victory for anyone who cares about the rule of law and clean air.”

In July, the four automakers announced that they had reached an agreement in principle with California on emissions standards that would be stricter than those being sought by the White House. The announcement came as an embarrassment for the Trump administration, which assailed the move as a “P.R. stunt.”

The Justice Department then opened an investigation into whether the four automakers violated federal antitrust laws by working together to reach their deal with California, on the grounds that the agreement could potentially limit consumer choice.

At the time Makan Delrahim, the assistant attorney general of the Justice Department’s antitrust division, wrote in an opinion article in USA Today that the investigation was not politically motivated. “Those who criticize even the prospect of an antitrust investigation should know that, when it comes to antitrust, politically popular ends should not justify turning a blind eye to the competition laws,” Mr. Delrahim wrote.