Airlines are under investigation by federal prosecutors for suspected collusion to keep flight fares high, just two years after the Justice Department approved a series of airline mergers with the goal to benefit customers. The department had tried to block the most recent merger, the 2013 joining of American Airlines and U.S. Airways, but ultimately agreed to let it proceed after the airlines made minor concessions, Fox News reports.

Letters were sent to airlines with the request to hand over to the prosecutor's office copies of documents from the last two years related to decisions and statements regarding the limiting of seat capacities for flight routes.

The Justice Department has also asked each airline to provide data of its carrying capacity both by region, and overall, since January 2010.

"We are investigating potential unlawful coordination among some airlines," said Emily Pierce, a Justice Department spokeswoman, according to New York Times.

American Airlines Group Inc., United Continental Holdings Inc., Delta Air Lines Inc., and Southwest Airlines Co., four of the largest U.S. airlines, confirm that they are part of the probe and are in full cooperation with the investigation.

JetBlue Airways Corp. and Alaska Air Group Inc. said the Justice Department hadn't contacted them as of Wednesday, Wall Street Journal reports.