According to the U.S. Department of Transportation, it will investigate Southwest Airlines flight cancellations that left passengers stranded at airports across America during a severe winter storm that claimed the lives of dozens.

Many airlines had to cancel flights because of the weather. Southwest was the worst affected. According to FlightAware tracking website, approximately 4,000 domestic U.S. flight cancellations were made Monday. 2,900 of these were Southwest’s. As of Tuesday morning, nearly 2,500 additional flights had been cancelled. This trend is likely to continue through Wednesday.

Jay McVay, spokesperson for Southwest, stated at a Houston press conference that cancellations have increased as severe weather systems move across the country. This has left flight crews and planes out-of-place.

He said, “So we’ve been running after our tails, trying desperately to catch up and get to safety, which was our number one priority as fast as possible.” “And that’s how we got to where we are today.”

People waited in long lines to rebook flights. On Twitter, the Department of Transportation stated that it was “concerned about Southwest’s unacceptable rate of cancelations and delays & reports of poor customer service.” It said that the department would investigate whether Southwest could have done any to address the cancellations or if the airline was following its customer service policy.

In an interview with The Wall Street Journal, Bob Jordan, Southwest CEO, stated that the airline would operate a little more than a third its normal schedule to allow crews to return to their destinations.

“We had a difficult day today. He said Monday evening that it is likely that we will have another difficult day tomorrow while we try to get out of this. “This is the biggest scale event I’ve ever witnessed.”