Southwest Airlines scrubbed thousands of flights again Tuesday in the aftermath of the massive winter storm that wrecked Christmas travel plans across the U.S., and the federal government said it would investigate why the company lagged so far behind other carriers.

A day after most U.S. airlines had recovered from the storm, Southwest called off about 2,600 more flights on the East Coast by late afternoon. Those flights accounted for more than 80% of the 3,000 trips that got canceled nationwide Tuesday, according to tracking service FlightAware.

And the chaos seemed certain to continue. The airline also scrubbed 2,500 flights for Wednesday and nearly 1,400 for Thursday as it tried to restore order to its mangled schedule.

At airports with major Southwest operations, customers stood in long lines hoping to find a seat on another flight. They described waiting hours on hold for help, only to be cut off. Some tried to rent cars to get to their destinations sooner. Others found spots to sleep on the floor. Luggage piled up in huge heaps.

The size and severity of the storm created havoc for many airlines, although the largest number of canceled flights Tuesday were at airports where Southwest is a major carrier, including Denver, Chicago Midway, Las Vegas, Baltimore and Dallas.

Spirit Airlines and Alaska Airlines both canceled about 10% of their flights, with much smaller cancellation percentages at American, Delta, United and JetBlue.

The cancellations – and the disparity between airlines – triggered a closer look at Southwest operations by the U.S. Department of Transportation, which called the rate of cancellations "unacceptable and dramatically higher than other U.S. carriers."

In a statement to Spectrum News, a spokesperson for the department said that Transportation Secretary Pete Buttigieg spoke with Southwest CEO Bob Jordan amid the mass cancellations.

Per the spokesperson, Buttigieg "conveyed that he expects the airline to live up to the commitments it has made to passengers, including providing meal vouchers, refunds, and hotel accommodations for those experiencing significant delays or cancelations that came about as a result of Southwest's decisions and actions."

"Southwest, as all airlines, is also obligated to provide a cash refund for passengers whose flights were canceled and decided not to travel," the DOT spokesperson told Spectrum News.

Buttigieg also spoke with union leaders who represent Southwest's flight attendants and pilots and told the secretary that "many flight attendants and pilots are stranded alongside passengers, sleeping on cots or having to book their own hotel rooms."

"He also conveyed to Southwest's CEO that he expects Southwest to do right by their pilots and flight attendants-and all their workers- in these situations," the spokesman said, adding: "The Department will take action to hold Southwest accountable if it fails to fulfill its obligations and we will stay engaged with Southwest Airlines to make sure the airline does not allow a situation like this to happen again."

President Joe Biden also assured Americans his administration is "working to ensure airlines are held accountable," adding: If you’ve been affected by cancellations, go to @USDOT’s dashboard to see if you’re entitled to compensation."

U.S. Sen. Maria Cantwell, who chairs the Senate Committee on Commerce, Science and Transportation, said that the panel will look into the Southwest cancellations.

"The problems at Southwest Airlines over the last several days go beyond weather," the Washington Democrat said in a statement. "The Committee will be looking into the causes of these disruptions and its impact to consumers.

"Many airlines fail to adequately communicate with consumers during flight cancellations," Cantwell added. "Consumers deserve strong protections, including an updated consumer refund rule."

Two prominent Senate Democrats, Sens. Ed. Markey, D-Mass., and Richard Blumenthal, D-Conn., released a joint statement demanding the airline refund passengers who have been impacted by the cancellations.

"Instead of a holiday spent celebrating with family and friends, passengers are sleeping in airports or desperately trying to reach customer service agents," they wrote. "For those travelers whose holidays have been ruined, there is no real way for Southwest to make this right. But the company can start by fairly compensating passengers whose flights were canceled, including not only rebooked tickets, ticket refunds, and hotel, meal, and transportation reimbursement, but significant monetary compensation for the disruption to their holiday plans.

"Southwest is planning to issue a $428 million dividend next year – the company can afford to do right by the consumers it has harmed," the lawmakers continued. "Southwest should focus first on its customers stranded at airports and stuck on interminable hold.

"As Southwest executives have acknowledged, the mass cancellations yesterday were largely due to the failure of its own internal systems," they continued. As such, those cancellations should be categorized as ‘controllable,’ and Southwest should compensate passengers accordingly.”

In a video posted on social media Tuesday night, Southwest CEO Bob Jordan apologized to the airline’s customers and employees impacted by the cancellations.

“I want everyone who is dealing with the problems we've been facing — whether you haven't been able to get to where you need to go or you’re one of our heroic employees caught up in a massive effort to stabilize the airline — to know is that we're doing everything we can to return to a normal operation,” he said. “And please also hear that I'm truly sorry.”

Jordan noted that Southwest is the largest carrier in the U.S. and said its network is ‘highly complex” and relies “on all the pieces, especially aircraft and crews, remaining in motion to where they're planned to go.”

“After days of trying to operate as much of our full schedule across the busy holiday weekend, we reached a decision point to significantly reduce our flying to catch up,” Jordan explained.

He added that Southwest is “processing refunds, proactively reaching out and taking care of customers who are dealing with costly detours and reroutes.”

The problems began over the weekend and snowballed Monday, when Southwest called off more than 70% of its flights.

That was after the worst of the storm had passed. The airline said many pilots and flight attendants were out of position to work their flights. Leaders of unions representing Southwest pilots and flight attendants blamed antiquated crew-scheduling software and criticized company management.

Southwest spokesman Jay McVay said the cancellations grew 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 told a news conference late Monday in Houston.

Casey Murray, president of the Southwest Airlines Pilots Association, said the airline failed to fix problems that caused a similar meltdown in October 2021.

“There is a lot of frustration because this is so preventable,” Murray said. “The airline cannot connect crews to airplanes. The airline didn’t even know where pilots were at.”

Murray said managers resorted this week to asking pilots at some airports to report to a central location, where they wrote down the names of pilots who were present and forwarded the lists to headquarters.

“I’m taking it to the highest level — that is how done we are,” said Lyn Montgomery, president of the Transport Workers Union representing Southwest flight attendants. “This is a very catastrophic event.”