Exhausted travelers sought passage by other means using different airlines, rental cars, or trains — or they’ve simply given up.

According to the FlightAware tracking service, more than 91% of all canceled flights in the U.S. early Wednesday were from Southwest, which has been unable to recover from ferocious winter storms that raked large swaths of the country over the weekend.

The operational systems of Southwest have been uniquely effected, so much so that the federal government is now investigating what happened at the Dallas carrier, which has frustrated its own flight and ground crews as well.

This week, with cancellations from other major airlines ranging from none to 2%, Southwest has canceled nearly 10,000 flights as of Wednesday and warned of thousands more Thursday and Friday, according to FlightAware.

In a video that Southwest posted late Tuesday, CEO Robert Jordan said Southwest would operate a reduced schedule for several days but hoped to be “back on track before next week.”

Jordan blamed the winter storm for snarling the airline’s “highly complex” network. He said Southwest’s tools for recovering from disruptions work “99% of the time, but clearly we need to double down” on upgrading systems to avoid a repeat of this week.

“We have some real work to do in making this right,” said Jordan, a 34-year Southwest veteran who became CEO in February. “For now, I want you to know that we are committed to that.”

The airline is now drawing unwanted attention from Washington.

Transportation Secretary Pete Buttigieg, who has criticized airlines for previous disruptions, said his agency would examine the causes of Southwest’s widespread cancellations and whether the airline was meeting its legal obligations to stranded customers.

“Because what we’re seeing right now, from the system and the flights themselves to the inability to reach anybody on a customer service phone line, it is just completely unacceptable,” Buttigieg told CBS early Wednesday.

In Congress, the Senate Commerce Committee also promised an investigation. Two Senate Democrats called on Southwest to provide “significant” compensation for stranded travelers, saying that the airline has the money because it plans to pay $428 million in dividends next month.