The aftermath from a massive winter storm caused nationwide chaos for travelers trying to catch a flight on the day after Christmas, especially those booked on Dallas-based Southwest Airlines.

As of 9 pm Central on Monday, December 26, more than 3,900 flights within, into, or out of the United States were canceled, according to the tracking site FlightAware.

Southwest Airlines accounted for the vast majority of those canceled flights and eventually apologized to passengers and employees for the "unacceptable" situation.

In many cases, stranded passengers were told it would be at least three or four days before they could be rebooked on another Southwest flight.

FlightAware said Southwest had 2,902 cancellations Monday — about 71 percent of its scheduled flights for the day and 10 times as many as any other major U.S. carrier. By about 12:20 pm Tuesday, Southwest had canceled another 2,500, or 63 percent, of its scheduled flights, as well as 30 percent of its Wednesday flights.

More than 200 Southwest flights from Dallas Love Field alone were canceled Tuesday, according to Flight Aware.

For comparison, Delta Airlines had the second-most cancellations Monday for a U.S. carrier with around 268, about 9 percent of its scheduled flights.

The U.S. Department of Transportation said Monday evening it was "concerned" by the high number of cancellations and reports of lacking customer service. The federal agency said it would be examining whether the "cancellations were controllable."

Southwest said Monday afternoon that with "consecutive days of extreme winter weather across our network behind us, continuing challenges are impacting our Customers and Employees in a significant way that is unacceptable. And our heartfelt apologies for this are just beginning."

An aviation expert told Dallas-Fort Worth TV station WFAA that Southwest Airlines was one of the hardest hit by the big winter storm last week.

"You’ve got a combination of weather, crews out of position," Steve Cosgrove, CEO of Dynamic Travel, told WFAA.

Issues are compounded by scheduling, a shortage in pilots and staff, and a limit to the hours they can work.

"Southwest runs a very tight schedule. They turn a plane in 20 minutes. You start getting planes and crews out of place, it’s tough to get them back in place," he says.

Another big reason Southwest was more impacted than other airlines is that it operates on a point-to-point system rather than a hub-and-spoke system, as this story explains.

Southwest also does not partner with any other airlines, making rebooking more challenging.

The U.S. Department of Transportation's Office of Aviation Consumer Protection has created a dashboard for travelers to check their rights, cancellations, and more.

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Read the full story and watch the video at KVUE.com. Stephanie Allmon Merry contributed to this report.