“We welcome the review as the data shows that the industry remains highly competitive with more people flying than ever before,” the airline said in a statement. “Demand has been enabled by a robust and competitive marketplace, in which capacity has been added and average fares have decreased.”

Luke Punzenberger, a United Airlines spokesman, confirmed that United had received the Justice Department letter. He said United was complying with the request for documents.

Delta said it had received the Justice Department’s letter and was fully cooperating. Southwest did not respond to a request for comment.

Antitrust lawyers and aviation analysts said that investigators would have to resolve a central question: whether the airline executives have talked so much publicly about discipline to appease Wall Street’s profit demands, or whether there is any smoking gun showing that airline executives have colluded privately.

“There have been about two years of conference calls in which Wall Street analysts have browbeat airline executives to either have discipline, or they will bust their recommendations on their stock,” said Robert W. Mann Jr., an aviation analyst in Port Washington, N.Y. “I don’t sense that the executives talk to each other. They actually hate each other, truth be told. But with so few of them left, there’s almost a natural oligopoly.”

He said that discount airlines like Spirit Airlines and Allegiant Air still offer rock-bottom fares. But among the major carriers, “there are no wounded competitors, which historically have been the ones who broke price and inflicted pain on the others,” he said.

Mr. Mann added that airline fares were “a little softer” so far in 2015 than they were last year. But baggage and other fees, like for upgraded seats or Internet access, have lifted airline profits, and increases in taxes on the tickets have left consumers paying more over all on many routes, he said.