(NewsNation) — Six airlines have paid more than half a billion dollars to customers who had a flight delayed or canceled since 2020, the Transportation Department announced in a news release.

Frontier Airlines, the only one based in the United States, was assessed the largest fine and ordered to pay $222 million in refunds — the most of any of the six. The airline was ordered to pay a $2.2 million fine on top of the refunds.

“When a flight gets canceled, passengers seeking refunds should be paid back promptly. Whenever that doesn’t happen, we will act to hold airlines accountable on behalf of American travelers and get passengers their money back,” said U.S. Transportation Secretary Pete Buttigieg. “A flight cancellation is frustrating enough, and you shouldn’t also have to haggle or wait months to get your refund.”

Frontier flies out of Cleveland Hopkins International Airport to 13 destinations, including Las Vegas, Denver, Fort Worth and Cancun, Mexico.

According to its federal consent decree, the company failed to refund tens of thousands of customers for flights that were “significantly changed,” meaning their departure or arrival were delayed three hours or longer, their airport changed, new connecting flights were added or the aircraft itself was downgraded.

Though the company issued flight credits which could be redeemed on the airline’s website before ultimately expiring, the system was not functioning for a portion of the refund period, according to the decree. The transportation department deemed that to be an unfair and deceptive practice.

Other airlines impacted include:

  • Air India – $121.5 million in required refunds paid and a $1.4 million penalty
  • TAP Portugal – $126.5 million in required refunds paid and a $1.1 million penalty
  • Aeromexico – $13.6 million in required refunds paid and a $900,000 penalty
  • El Al – $61.9 million in required refunds paid and a $900,000 penalty
  • Avianca – $76.8 million in required refunds paid and a $750,000 penalty

The enforcement action is part of the department’s work to crack down on flight delays and cancellations, which plagued travelers this summer after air travel rebounded from pandemic lows. Airlines are required under U.S. law to refund customers if the flight is canceled or significantly changed and the passenger does not wish to accept an alternative, such as a voucher.

The penalties assessed to the airlines were for delays in issuing refunds. The payments will be made to the U.S. Treasury department, according to the news release.