Washington (CNN) The Justice Department has sent subpoenas to several major airlines as part of an investigation into "possible unlawful coordination" to limit capacity increases, and thereby keeping ticket prices high, a department spokeswoman said Wednesday.

The department sent civil investigative demand letters to airlines on Tuesday, Justice spokeswoman Emily Pierce said.

At issue is whether different airlines are coordinating to keep capacity -- the number of seats available on planes -- stable, having the effect of keeping ticket prices high.

How the airlines have coordinated isn't immediately clear, but among the evidence that investigators is considering includes public comments by airline executives and industry analysts.

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In recent years, the Justice Department allowed two major airline mergers to take place -- including United and Continental, as well as American and U.S. Airways. Industry groups have cited the merging of airlines as an example of the business has gotten tougher for consumers.

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