The Biden administration says it is examining the four largest U.S. airline frequent-flyer programs and how they devalue points that consumers have earned and frequently change the number of points or miles needed to book flights.

Transportation Secretary Pete Buttigieg wrote to the CEOs of American, Delta, Southwest and United on Thursday, asking each for a report on policies, fees and other features of their loyalty program.

Consumers often complain that airlines raise the number of points needed to earn a free flight and limit the number of seats that can be purchased with points.

Buttigieg said loyalty programs bring value to consumers, and people count on them to pay for vacations and trips to visit family.

"But unlike a traditional savings account, these rewards are controlled by a company that can unilaterally change their value," he said in a statement issued by the Transportation Department. "Our goal is to ensure consumers are getting the value that was promised to them, which means validating that these programs are transparent and fair."

Frequent-flyer programs were once based on the number of flights taken or miles flown. In recent years, however, they have been fueled by spending that consumers conduct using airline-branded credit cards. Income from credit card issuers has become an important source of airline revenue.