Consumers filed nearly 20,000 complaints about airline service in April 2020 — an increase of more than 1,500 percent over the same period last year, according to new data released by the U.S. Transportation Department on Friday.

The dramatic increase may seem counterintuitive, because it comes at a time when so few people are flying. But a closer look at the data reveals why consumers are so unhappy with U.S. airlines: refunds.

Or in many cases, the inability of consumers to get refunds for trips they canceled because of the novel coronavirus.

Of the nearly 20,000 complaints filed in April, nearly 90 percent concerned refunds. By comparison, consumers filed just over 1,200 complaints in April of last year. The vast majority of those were about flight problems, including cancellations and delays. Only 98 concerned refunds.

United recorded the highest number of complaints. American Airlines ranked second.

Airlines for America, the airline trade group, said: “U.S. airlines comply with all federal laws and regulations. Accordingly, refunds are issued to passengers when flights are canceled by the airline. Throughout the pandemic, U.S. airlines have updated travel policies to offer increased flexibility for customers, and the Department of Transportation’s website offers clarity for consumers with questions.”

The carriers’ reluctance to provide refunds has drawn particular outrage because airlines received more than $50 billion in bailout funds as part of the $2 trillion coronavirus relief package known as the Cares Act.

Southwest, United and Spirit airlines have all been sued by consumers outraged that carriers offered them vouchers or credits for future trips rather than cash.

The dramatic increase in complaints has also prompted warnings from the Transportation Department. In May, the department issued a second enforcement notice reminding carriers of their obligations under consumer protection laws. Although the notice does not guarantee that travelers will receive refunds, it offers them additional information about their rights — and reminds airlines that they could face consequences if they refuse to comply.

In a statement that accompanied the notice, officials said the department will be monitoring airlines’ refund practices and would issue letters or fines “as necessary.” Until then, it would give airlines “an opportunity to come into compliance.”

In March, as the issue was heating up, a group of nine senators, including Richard Blumenthal (D-Conn.), wrote to airlines saying that the pandemic was putting an “enormous financial strain on millions of Americans,” and that carriers had “a moral responsibility to provide real refunds, not travel vouchers” to consumers.

Based on information received from airlines, the lawmakers estimated that carriers were potentially holding on to as much as $10 billion in consumer cash.

Airlines have shifted some policies in response to consumer complaints, but some travelers still report running into problems when they have asked for their money back. Airline executives said they are trying to accommodate consumers, but are struggling to stay afloat in the midst of the worst downturn in industry history.

Friday’s report also showed how dramatically airlines have cut their operations. Airlines scheduled fewer than half the number of flights they did in March and ultimately ended up canceling 41 percent of the ones that were scheduled within a week of their scheduled departure. It was the highest number of cancellations on record, exceeding the number canceled in September 2001, the month of the 9/11 terrorist attacks.

In the end, carriers flew 194,390 flights in April, the lowest since February 1994, when they operated 370,027 flights.