Transportation Department to review Southwest's mass flight cancellations

Thousands more flights were canceled on Monday as the effects of winter storms and operational challenges continued to impact holiday travel.

The big picture: The Department of Transportation tweeted that it would look into the actions of Southwest, which accounted for a large amount of the more than 3,900 flight cancelations and 8,200-plus delays within, into or out of the U.S. as of late Monday, per FlightAware data.