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The US Department of Justice will appeal the court ruling that allowed AT&T to purchase Time Warner Inc.

AT&T completed the merger after getting a favorable ruling from a judge at the US District Court for the District of Columbia last month. The Trump administration’s Justice Department did not seek a stay of the ruling, so AT&T was able to take ownership of Time Warner. But the DOJ is appealing the judge’s ruling to the United States Court of Appeals for the District of Columbia Circuit, the DOJ said in a court filing today.

A court could theoretically force AT&T and Time Warner to reverse the merger. AT&T said it would maintain some separation between its old and new business units in a post-verdict letter to the DOJ. That separation might make undoing the merger logistically easier if the DOJ wins its appeal.