The Justice Department is investigating whether some of the biggest airlines in the United States colluded to keep airfares high.

Justice Department spokesperson Emily Pierce told The Associated Press investigators are looking into “possible unlawful coordination by some airlines.” Representatives from Delta Air Lines, American Airlines, United Airlines and Southwest Airlines confirmed they are among those carriers under investigation and are complying with all DOJ requests, according to NBC News.

The antitrust investigation is questioning whether major airlines are illegally coordinating with one another to limit capacity, leading to keeping fares high to the detriment of passengers. The DOJ is focusing on whether the carriers signaled to one another how quickly they would add new flights, routes and extra seats through public statements or private statements to market analysts.

In response, airline industry trade group Airlines for America issued a statement detailing the spirit of competition of their members, saying it is “confident that the Justice Department will find what we know to be true: Our members compete vigorously every day, and the traveling public has been the beneficiary.”

Since the start of a series of mergers in 2008 that led to America, Delta, Southwest and United controlling more than 80 percent of the seats in the domestic travel market, the average domestic airfare increased 13 percent from 2009 to 2014, according to Bureau of Transportation statistics.

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