Transport secretary Pete Buttigieg sent letters to American, Delta, Southwest and United Airlines, ordering them to provide records and submit reports answering detailed questions on their rewards programmes.

The department is reviewing how consumers are “impacted by the devaluation of earned rewards, hidden or dynamic pricing, extra fees, and reduced competition and choice.”

Reuters first reported in December that DOT had begun scrutinising the frequent flyer programmes of major US airlines for potential deceptive or unfair practices.

DOT and the Consumer Financial Protection Bureau held a public hearing in May on airline loyalty programmes, while some lawmakers have also scrutinised the programmes.

Buttigieg said the department is seeking “more specific data to better understand the largest rewards programmes and identify potential competition or consumer protection issues or risks.”

Customers often complain airlines have been raising the bar to earn perks associated with these programmes. The US Congress last year considered but did not mandate requiring airlines to provide 90 days’ notice before devaluing frequent flyer programme points.

Trade group Airlines for America, however, said US airlines are transparent about frequent flyer programmes “and policymakers should ensure that consumers can continue to be offered these important benefits.”

US carriers relied on these programmes, which have tens of millions of members, for revenue and to raise funds during the Covid-19 pandemic when travel demand plunged.

Loyalty programmes of Delta, United and American were each valued more than US$20 billion last year, according to consulting firm On Point Loyalty.

Delta said it has received the DOT’s inquiry and will respond.

Southwest said its flexible rewards programme benefits have resulted in the carrier doubling the industry average of seats booked with points, while United and American declined comment.