The U.S. Department of Transportation is investigating the Southwest Airlines holiday travel debacle, which left thousands of travelers stranded for days. The investigation comes as the airline reported a $220 million loss last quarter and further losses in the first quarter.

Southwest canceled more than 16,700 flights over several days in late December. While a massive winter storm caused the initial cancellations, the company's outdated software systems turned what should have been a normal problem into a snowballing disaster that lasted for days after other airlines had resumed their usual operations.

The department's investigation will look into whether Southwest made unrealistic flight schedules, "which under federal law is considered an unfair and deceptive practice," according to a department spokesperson.

"DOT has made clear to Southwest that it must provide timely refunds and reimbursements and will hold Southwest accountable if it fails to do so," the department spokesperson said.

The flight cancellations cost the company about $800 million, according to Bob Jordan, the airline's president and chief executive officer.

About half of those losses come directly from the flight cancellations. The rest largely come from compensating customers who bought tickets on other airlines and dispensing extra frequent flier points, which are worth about $300 per passenger.

Passengers and employees alike were frustrated by the company's lack of communication during and immediately after the cancellations.

In the report announcing the losses, Jordan apologized to customers and employees, saying the company has "swiftly taken steps to bolster our operational resilience and are undergoing a detailed review of the December events."

Fewer people are booking with Southwest due to the December 2022 disaster. The company is expecting to lose over $300 million in revenue in the first quarter, though it reports that booking trends are improving.