02:58 PM EDT, 09/05/2024 (MT Newswires) -- (Update with responses from Southwest Airlines ( LUV ), American Airlines ( AAL ) and Airlines for America.)

The US Department of Transportation launched an investigation on Thursday into the rewards programs of the four largest US airlines.

US Secretary of Transportation Pete Buttigieg sent letters to Delta Air Lines ( DAL ) , American Airlines ( AAL ) , United Airlines (UAL) and Southwest Airlines ( LUV ) ordering them to provide records and submit reports with information on their rewards programs, policies and practices.

The regulator's investigation is concentrated on the ways participants in airline rewards programs are affected by the devaluation of earned rewards, hidden dynamic pricing, additional fees and diminished competition and choice, the Department of Transportation said.

Delta confirmed receipt of Department of Transportation's enquiry to MT Newswires and said it will respond to it accordingly.

A Southwest ( LUV ) spokesperson told MT Newswires that its rewards program has no expiration date and its success has translated into double the industry average of seats booked with points. Points can be redeemed for flights, hotels, car rentals and more.

American Airlines ( AAL ) referred MT Newswires to Airlines for America, which said: "US carriers are transparent about these programs, and policymakers should ensure that consumers can continue to be offered these important benefits."

United did not immediately reply to requests for comment.

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