Southwest Airlines Executive is set to explain the holiday meltdown to Congress

Congress wants to know what led to Southwest Airlines' massive meltdown around Christmas.

Congress is set to receive new evidence Thursday of internal chaos at Southwest Airlines over the Christmas holiday meltdown.

The Senate Commerce committee is set to question Southwest executive Andrew Watterson, alongside Southwest pilot union president Casey Murray, Sharon Pinkerton of the Airlines for America trade group, Paul Hudson of Flyers’ Rights, and economist Clifford Winston of The Brookings Institution.

Senators are investigating the scheduling issues in late December, when the airline canceled more than 16,000 flights. Some 2 million passengers were stranded and there were mountains of unclaimed luggage at airports across the nation.

The Pilots Association will introduce new evidence, including a photo of a message sent during the meltdown to a cockpit computer.

Southwest declined to comment on the union’s allegations ahead of the hearing.