The US Department of Transportation (DOT) Tuesday promised to look into massive flight cancellations and delays by Southwest Airlines, the world’s largest low-cost carrier, amidst an unprecedented winter storm. According to FlightAware, a flight tracking website, 2,694 flights or 64 percent of Southwest’s scheduled flights were canceled on Tuesday. Another 25 percent of Southwest’s schedule was delayed.

Recent days saw an unprecedented winter storm, which meteorologists described as a “bomb cyclone” or “bombogenesis.” Temperatures dropped by 30 degrees Fahrenheit in 18 hours, leading to record-low temperatures, heavy snowfall and severely reduced visibility.

This adversely affected air travel in the US. While most airlines recovered, passengers traveling with Southwest continued to face delays and cancellations. Many passengers took to Twitter to vent their frustrations. The hold times at the Southwest customer service line reportedly reached four hours.

President Joe Biden tweeted that his administration was working to ensure airlines are “held accountable” for the cancellations and delays. As a part of the administration’s response, Secretary of Transportation Pete Buttigieg said in an CNN interview that he spoke to the CEO of Southwest Airlines, Bob Jordan, to make sure the company provided refunds, meals and hotel fares to those affected. Buttigieg said the events were a “meltdown” and placed blame on Southwest.

Jordan apologized on Twitter in a video, blaming the crisis on outdated flight management software that the airline neglected to upgrade. Jordan further stated that Southwest is confident the crisis will be resolved by next week.

Southwest’s website is currently allowing passengers traveling through January 2 to rebook their flights without additional charges.