‘Unacceptable’ Southwest Airlines to fall under federal probe after it cancels nearly 75% of all flights on Monday. ‘We’ve been chasing our tails.’

Many airlines were forced to cancel flights due to the weather, but Southwest was by far the leader in canceled flights. About 4,000 domestic U.S. flights were cancelled Monday, according to the tracking website FlightAware, and 2,900 of those were Southwest’s.

Other major airlines, including American, United, Delta and JetBlue, suffered cancellations rates of between none and 2%. The cancellation rate at Southwest Airlines was 62%, according to FlightAware.

Southwest spokesman Jay McVay said at a press conference in Houston that cancellations snowballed as storm systems moved across the country, leaving flight crews and planes out of place

“So we’ve been chasing our tails, trying to catch up and get back to normal safely, which is our number one priority as quickly as we could,” he said. “And that’s exactly how we ended up where we are today.”

More than 2,800 more flights had already been canceled in the U.S. as of 7 a.m. Tuesday and problems are likely to continue at least into Wednesday.

Passengers stood in long lines trying to rebook their flights. The Department of Transportation said on Twitter that it was “concerned by Southwest’s unacceptable rate of cancellations and delays & reports of lack of prompt customer service.” The tweet said the department would look into whether Southwest could have done anything about the cancellations and whether the airline was complying with its customer service plan.

Southwest CEO Bob Jordan told The Wall Street Journal in an interview that the airline would operate just over a third of its usual schedule to allow crews to get back to where they needed to be.

“We had a tough day today. In all likelihood we’ll have another tough day tomorrow as we work our way out of this,” he said Monday evening. “This is the largest scale event that I’ve ever seen.”