The Department of Transportation has opened an investigation into scheduling problems at Southwest Airlines that caused a late December meltdown and left thousands of travelers stranded. The probe will focus on whether the company knowingly sold more flights than it could carry out, according to The Wall Street Journal. “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,” a spokeswoman told the newspaper. Southwest denied the charges Wednesday, saying in a statement that its plans were sustainable and realistic. “Our holiday flight schedule was thoughtfully designed and offered to our Customers with the backing of a solid plan to operate it, and with ample staffing,” the company said, adding that it would cooperate fully with investigators.