The U.S. Department of Justice said on Wednesday it is investigating whether U.S. airlines worked together illegally to keep airfares high by signaling plans to limit flights.
The Justice Department wrote to major U.S. air 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 four confirmed receipt of the regulator's letter and said they are cooperating fully with the investigation. News of the probe sent the Dow Jones U.S. airlines index down 2 percent.
Shares of the U.S. 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.