The US Department of Justice is investigating whether US airlines worked together illegally to keep airfares high by limiting flights.
The Justice Department wrote to major US carriers demanding that they detail decisions to limit the number of seats they offer, and what they've said about those plans to investors, securities analysts and the public.
Airlines contacted have been asked to provide "available seat miles on a regional and system wide basis" back to January 2010 and a raft of other data.
The top four US carriers American, Delta, United and Southwest control around 80 percent of the domestic air travel market.
The four confirmed receipt of the regulator's letter and said they are cooperating fully with the investigation.
Shares of the US carriers have gyrated in recent weeks as investors questioned whether they were planning to add capacity at a pace faster than overall economic growth, which could put downward pressure on fares.
Southwest fuelled investor jitters about declining profit margins in May when it unveiled plans to boost capacity by 8 percent this year from 2014, although it later revised the expected increase to 7 percent.
But carriers have started taking flights off upcoming schedules and postponing aircraft deliveries in response to Wall Street concerns that adding more flights and seats could erode fares and margins.
Mergers, new fees imposed on passengers and caution about adding capacity have boosted US airline earnings after a decade of bankruptcies following the September 11, 2001 attacks. In the past year, tumbling oil prices have helped the carriers post billion-dollar profits.
The investigation focusing on whether the big US carriers are colluding domestically comes as some of the same airlines complain that foreign rivals are competing unfairly on some overseas routes.
US carriers have asked the Obama administration to freeze access to US airports by three Gulf carriers for allegedly receiving state subsidies. The Gulf airlines deny that they have received subsidies in violation of trade agreements.
The US airlines also have fought plans by low-cost Norwegian Air Shuttle to expand its US flights under an Irish subsidiary, with claims that it will undermine wages and working standards.
Consumer advocates and some politicians praised the Justice department action focusing on domestic fares.
"This investigation must be tireless and timely to save consumers from the onslaught of price increases in summer fares," US Senator Richard Blumenthal said in a statement.
The airlines and their defenders say ticket prices have fallen in 2015 while capacity this summer has reached a post-recession high.
"Our members compete vigorously every day, and the travelling public has been the beneficiary," trade group Airlines for America said in a statement.
Separately, Connecticut's attorney general sent letters to the four carriers last week asking whether they have coordinated prices, citing recent statements at an industry conference held last month in Miami.