By DAVID KOENIG, SCOTT MAYEROWITZ and ERIC TUCKER

The U.S. government is investigating possible collusion among major airlines to limit available seats, which keeps airfares high, according to a document obtained by The Associated Press.

The civil antitrust investigation by the Justice Department appears to focus on whether airlines illegally signaled to each other how quickly they would add new flights, routes and extra seats. A letter received Tuesday by major U.S. carriers demands copies of all communications the airlines had with each other, Wall Street analysts and major shareholders about their plans for passenger-carrying capacity, or "the undesirability of your company or any other airline increasing capacity."

American Airlines, Delta Air Lines, Southwest Airlines and United Airlines all said they received a letter and are complying. Several smaller carriers, including JetBlue Airways and Frontier Airlines, said they had not been contacted by the government. The department had tried to block the most recent merger, the 2013 joining of American Airlines and US Airways, but ultimately agreed to let it proceed after the airlines made minor concessions.

Guests:

Charlie Leocha, Chairman, Travellers United (formerly Consumer Travel Alliance) - a nonprofit, nonpartisan organization; TravellersUnited.org

Marc Scribner, Fellow in Transportation Studies, Competitive Enterprise Institute