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The Department of Transportation (DOT) is probing whether “unrealistic scheduling” played a role in the mass flight delays and cancellations from Southwest Airlines during the holidays last year.  

“DOT is in the initial phase of a rigorous and comprehensive investigation into Southwest Airlines’ holiday debacle that stranded millions … [and] probing whether Southwest executives engaged in unrealistic scheduling of flights which under federal law is considered an unfair and deceptive practice,” a department spokesperson told The Hill Thursday.  

“DOT will leverage the full extent of its investigative and enforcement power to ensure consumers are protected and this process will continue to evolve as the Department learns more,” the spokesperson said. 

Winter weather delayed and canceled flights across the country during the busy holiday travel season, but Southwest saw far more sweeping problems, canceling nearly 17,000 flights over the last 10 days of the year. 

Shares of the company fell, and Southwest took an $800 million hit for the cancellations.

The Hill has reached out to Southwest for comment.

Lawmakers have pledged to probe the airline for the holiday chaos. Sen. Maria Cantwell (D-Wash.), the chairwoman of the Senate Commerce, Science and Transportation Committee, said Southwest’s disruptions “go beyond weather” and indicate a broader problem. 

Southwest promised reimbursement for passengers affected by cancellations. The Transportation Department said Thursday that it’s “made clear” to the airline company that it has to provide those refunds in a timely schedule and “will hold Southwest accountable if it fails to do so.” 

The Dallas Morning News first reported the Transportation Department’s latest remarks.