Frequent assurances by U.S. airline executives that they would maintain “discipline” when adding seats on competitive routes were a red flag for antitrust regulators on alert for collusion between the four largest air carriers, antitrust experts said Thursday. The U.S. Justice Department disclosed Wednesday that it has demanded details from the airlines about their communications on capacity with analysts, investors, the public and each other. Concern about coordination among the big air carriers was one reason the Justice Department sued in 2013 to stop U.S.