Dallas-based Southwest Airlines faces a federal investigation into whether it violated its own legally required customer service plan amid a blizzard of flight cancellations that ruined plans and angered travelers over the Christmas holiday.

In a statement late Monday, officials at the U.S. Department of Transportation called the service meltdown, which resulted in the cancellation or delay of most of the carrier’s flights over the holiday weekend, “disproportionate and unacceptable.”

As Winter Storm Elliott started to wreak havoc on a large chunk of the U.S., the vast majority of canceled flights across the nation were operated by Southwest Airlines. And air travelers’ woes are likely to continue this week.

“USDOT is concerned by Southwest’s unacceptable rate of cancellations and delays & reports of lack of prompt customer service,” the agency posted on Twitter on Monday evening. “The Department will examine whether cancellations were controllable and if Southwest is complying with its customer service plan.”

Transportation Secretary Pete Buttigieg tweeted shortly after that he was “tracking [the issue] closely” and would have more to say about this Tuesday.

Late Tuesday afternoon, the DOT said on Twitter that Buttigieg had spoken “with union leaders and the CEO of Southwest Airlines to convey the Department’s expectation that Southwest meet its obligations to passengers and workers and take steps to prevent a situation like this from happening again.”

Southwest officials said in a message to employees, obtained by the The Dallas Morning News on Tuesday, that staffing issues were a large part of the reason the planes were being grounded after pilots and other staff couldn’t get to the airports where they were needed.

Some travelers were told that no new bookings were being made before the new year.

In a statement to The Dallas Morning News, a spokesperson for the airline — the largest in 23 of the nation’s top 25 travel markets — said that it was slashing flights and halting ticket sales for the coming days while it recovers.

“Due to our limited schedule and large number of re-accommodations, inventory available to book flights across our network is very low, but we are still operating flights,” Southwest spokesperson Chris Perry said in a statement.

Mike Santoro, vice president of the pilots union for the airline, told CNN the core of the issue was an outdated scheduling system that was already overwhelmed before the storm became the catalyst for the current problems. Those technology problems, he said, have left pilots and crews stranded in cities across the country looking for accommodations, unable to get through to airline officials to find out where they — or the planes they’re scheduled to staff — are supposed to be.

“We don’t know where we are, they don’t know where the airplanes are, and it’s frustrating,” Santoro said. “We are tired of apologizing for Southwest. Our hearts go out to all of the passengers.”

On Monday, 71% of Southwest flights were canceled, two-thirds of the overall number of canceled flights from any carrier into and out of the U.S. that day — even as Winter Storm Elliott, which barreled in Friday, began to abate.

On Tuesday, the airline scrubbed 64% of its flights, representing nearly 90% of cancellations from all carriers into and out of the country, according to the online airline tracker FlightAware.

The carrier has already canceled 62% of its flights for Wednesday — accounting for all but 33 of more than 2,500 canceled U.S. flights that day.

Southwest officials issued “heartfelt apologies” Monday that they said were “just beginning” as challenges they also described as “unacceptable” are expected to continue this week.

Officials said in the statement that the severe winter weather caused “wide-scale disruption” and that the airline is working at full capacity to restabilize service.

“We anticipate additional changes with an already reduced level of flights as we approach the coming New Year holiday travel period,” the statement read. “We’ll work to make things right for those we’ve let down.”

That may come as cold comfort for travelers who spent the entire holiday weekend in Austin-Bergstrom International Airport, who were separated from stacks of luggage at Houston Hobby Airport and who were dropped off at Love Field in Dallas — the Southwest hub — without having been notified that their flights were canceled. On Christmas Day, more than 100 flights were delayed or canceled at the San Antonio International Airport.

The chaos left some travelers sleeping on airport floors, waiting for hours in lines and overwhelming hotels in the scramble to find last-minute accommodations.

But federal officials are limited on what they can do beyond a harshly worded social media callout for an airline to enforce its customer services commitments. Those commitments are not required for an airline to keep its license but were established in 2011 law as a sort of traveler’s bill of rights that airlines are supposed to enforce on themselves.

Airlines can be heavily fined by DOT for breaking their rules on how to respond to long delays of planes filled with passengers sitting for hours on the tarmac, but it is unclear yet if those were among the major issues since most flights were canceled altogether.

“It’s posturing on DOT’s part,” said Charlie Leocha, president of Travelers United, which advocates for air travelers and has pushed for stronger ways to enforce airline customer service plans and other requirements.

Typically, he said, Southwest is a “customer service champion” as the only airline with no change fees, with credits that don’t expire and without charges for up to two bags, with few limitations.

The weather has been unprecedented, and as the largest carrier in the U.S., he added, Southwest is bound to have higher cancellation numbers during busy flight seasons.

“This weather situation is overwhelming. I can’t remember a period with such cold temperatures and an amazing amount of snow,” he said. “It is a shame that the real customer service champion has found itself in the crosshairs. All airlines have the same problems.”

This article originally appeared in The Texas Tribune, a member-supported, nonpartisan newsroom informing and engaging Texans on state politics and policy. Learn more at texastribune.org.

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Karen Brooks Harper, The Texas Tribune

Karen Brooks Harper is a general assignments reporter for The Texas Tribune.