Prosecutors at the US department of justice have begun an investigation into possible collusion among the airlines to limit seating.
The move comes just two years after the same department approved a wave of mergers, saying the combination would benefit consumers.
Officials have requested documents related to decisions made in the past two years concerned with limiting capacity on flight routes
In November 2013 the department cleared the way for the merger of American Airlines and US Airways.
The merger had initially been opposed by the department of justice.
That move meant more than three quarters of air traffic is now carried by four airlines – American United Airlines, Delta Air Lines, and Southwest Airlines.
American Airlines said it would cooperate with the investigation.
“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.”