WASHINGTON, Sept 5 (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 Delta Air Lines, Southwest

Airlines ( LUV ), 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.