A U.S. Senate panel on Tuesday said it was stepping up its investigation into billions of dollars in annual airline fees for baggage, seat selection, ticket changes and other services, demanding airline responses by next month.

Democratic Senator Richard Blumenthal, who chairs the Permanent Subcommittee on Investigations, first wrote CEOs of American Airlines, United Airlines, Delta Air Lines, Spirit Airlines and Frontier Airlines in November seeking answers.

In new letters, Blumenthal said the limited responses to date from the air carriers "raise serious questions" about the willingness of carriers to cooperate. He demanded airlines provide outstanding information and records sought by April 2 or the committee will "consider alternate means to obtain information needed to conduct its inquiry."

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The airlines declined to comment or did not comment on the letter.

Blumenthal noted that between 2018 and 2022, total revenue across major U.S. airlines from baggage fees increased from $4.9 billion to $6.8 billion. He cited a report by a travel consultancy that found that eight leading U.S. airlines in 2022 collected an estimated $4.2 billion in fees for seat selection.

Last month, United, American Airlines and JetBlue Airways all hiked checked baggage fees for customers traveling in North America, the first time in four years.

The committee also wants transcribed interviews with senior airline executives by late next month on fees.

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In September 2022, the U.S. Transportation Department (USDOT) proposed requiring airlines to disclose fees for baggage, ticket changes and family seating the first time an airfare is displayed. It also proposed rules in 2021 to require airlines to refund fees for significantly delayed bags and refunds for services such as onboard Wi-Fi when they do not work.

Airline CEOs in 2018 lobbied against bipartisan legislation to mandate "reasonable and proportional" baggage and change fees and convinced Congress to drop the plan.