The US has decided that Southwest's customer service failed during flight cancellations last winter

The federal government may be preparing to penalize Southwest Airlines for thousands of flight cancellations that affected more than 2 million travelers last December. Southwest disclosed the potential fine on Monday, and said it couldn't estimate how large it might be. The airline says the U.S. Transportation Department notified the company last Friday that it determined Southwest failed to provide adequate customers service during the meltdown, which started with a winter storm before Christmas and spun out of control. Southwest says in a regulatory filing that it also faces possible costs from lawsuits filed by customers and shareholders.