CHICAGO, IL - SEPTEMBER 24: Passengers wait in line to go through security screening at O'Hare International Airport's Terminal 1 shortly after the terminal was reopened on September 24, 2014 in Chicago, Illinois. The ticketing and baggage claim areas of the terminal were evacuated for nearly two hours after a after an unattended bag was discovered around 9:30 this morning. (Photo by Scott Olson/Getty Images)
Scott Olson/Getty Images
CHICAGO, IL - SEPTEMBER 24: Passengers wait in line to go through security screening at O'Hare International Airport's Terminal 1 shortly after the terminal was reopened on September 24, 2014 in Chicago, Illinois. The ticketing and baggage claim areas of the terminal were evacuated for nearly two hours after a after an unattended bag was discovered around 9:30 this morning. (Photo by Scott Olson/Getty Images)
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(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.

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.

“If not for the radical consolidation we have seen in the airline industry in the last few years, we probably would not even be having this conversation. Now that four carriers control 85 percent of domestic routes, ‘collusion’ is a thought that’s constantly going to be in the back of the minds of federal regulators,” U.S. Travel Association President and CEO Roger Dow said in a statement.

A United Airlines spokesperson confirmed to CNN that the airline has received a letter from the DOJ “and we are complying with their requests.”

American Airlines said they also had received notice from the Justice Department.

“We welcome the review … Demand has been enabled by a robust and competitive marketplace in which capacity has been added and average fares have decreased,” a spokesperson for American Airlines said. “We will cooperate fully with the investigation and demonstrate that the last two years have presented an entirely new competitive landscape that has greatly benefited air travel consumers.”

A spokesperson from Southwest Airlines confirmed receiving notice from Justice and said, “We’ll cooperate fully in answering any questions the (Justice Department) has of us.”

JetBlue said they have not been contacted for the investigation.

The Associated Press first reported on the Justice Department investigation.

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