WASHINGTON (Reuters) – The U.S. Transportation Department said on Thursday it is opening an inquiry into the four largest U.S. air carrier rewards and frequent flyer programs to ensure consumers do not face unfair, deceptive, or anticompetitive practices.

Transportation Secretary Pete Buttigieg sent letters to American Airlines (NASDAQ:) Delta Air Lines (NYSE:), Southwest Airlines (NYSE:), and United Airlines ordering them to provide records and submit reports answering detailed questions on their rewards programs as the department reviews how consumers are “impacted by the devaluation of earned rewards, hidden or dynamic pricing, extra fees, and reduced competition and choice.”

The airlines did not immediately comment.