Southwest Airlines Co. (NYSE:LUV) shares plunged nearly 6% on Tuesday as the company continues to cancel flights due to the winter storm. According to FlightAware data, Southwest has canceled nearly 3,000 flights, representing at least 70% of its schedule for the day. This is significantly more than any other airline, with Delta canceling 265 flights as of Monday night.

The U.S. Transportation Department has expressed concern about Southwest's "disproportionate and unacceptable rate of cancellations and delays" and has indicated that it will examine whether the cancellations were controllable and whether the company is complying with its customer service plan and other relevant rules.

Southwest has apologized for the disruptions and acknowledged that the extreme winter weather has had a significant impact on customers and employees.