WASHINGTON — A top Southwest Airlines executive is expected to tell Congress that «we were wrong» at a hearing Thursday that will explore mass flight cancellations that have left thousands of customers stranded for days over the holidays.

«Let me be clear: We were wrong. In retrospect, we didn’t have enough operating resilience over the winter,» Southwest COO Andrew Watterson will say, according to prepared remarks obtained by NBC News ahead of the Senate Commerce, Science hearing. and Transportation Committee.

The hearing is scheduled to begin around 10:30 am ET.

Southwest has come under intense scrutiny after it canceled thousands of flights in December, leaving many passengers struggling during the hectic holiday travel season. While other airlines had to disrupt services due to winter conditions, none were affected like Southwest.

The Department of Transportation said in January that it had opened an investigation into the collapse to determine «whether Southwest executives engaged in unrealistic flight scheduling that, under federal law, is considered an unfair and deceptive practice.»

«The American people have many questions about the Southwest debacle in December that left passengers stranded or unable to be with loved ones over the holidays,» said Commerce Committee Chair Sen. Maria Cantwell, D-Wash. in a statement issued before the hearing. «We’re going to ask for answers to those questions. I’m interested in hearing testimony from the pilot that this debacle could have been avoided if Southwest had made investments earlier.»

Capt. Casey A. Murray, president of the Southwest Airlines Pilots Association, will testify at Thursday’s hearing, along with Watterson; Paul Hudson, president of flyers rights group Flyers Rights; Sharon Pinkerton, an executive with Airlines for America, the trade group that represents airlines. Dr. Clifford Wilson, a senior fellow at the Brookings Institution, is also expected to testify remotely.

Cantwell also held a panel Wednesday with voters affected by Southwest’s problems.