(NEW YORK) — Southwest Airlines Chief Operating Officer Andrew Watterson faced lawmakers Thursday in a highly-anticipated Senate Commerce Committee hearing to answer for the airline’s historic holiday meltdown.

“Let me be clear: we messed up,” Watterson testified. “In hindsight, we did not have enough winter operational resilience.”

The largest domestic airline in the U.S., Southwest canceled more than 16,000 flights over an 11-day period at the end of December due to a combination of severe winter weather, staffing shortages and technology issues, the company said. Thousands were left stranded in airports across the country instead of at home for the holidays.

Lawmakers want the company to explain the massive disruption at Thursday’s Senate hearing, titled “Strengthening Airline Operations and Consumer Protections.”

“The American people have a lot of questions about the Southwest debacle in December that left passengers stranded or unable to be with loved ones over the holidays,” said Sen. Maria Cantwell, D-Wash., Chair of the Senate Committee on Commerce, Science and Transportation on Wednesday. “We’re going to ask for answers to those questions. I’m interested in hearing the pilot’s testimony that this debacle could have been avoided if Southwest had made investments sooner.”

In addition to Watterson, Southwest Airlines Pilots Association President Captain Casey Murray, Sharon Pinkerton, a senior official with Airlines for America, and Paul Hudson of Flyers’ Rights, a passenger advocacy organization, testified.

Murray said that he and other Southwest pilots saw the meltdown coming but that company leaders ignored their warnings.

“For years, our pilots have been sounding the alarm about Southwest’s inadequate crew scheduling technology and outdated operational processes. Unfortunately, those warnings have been summarily ignored by Southwest leaders,” he said.

He said his goal in participating in the hearing is to help ensure it never happens again.

“Today’s hearing is not to say we told you so. Right doesn’t make our pilots feel any more secure. Our hearts are broken. The December 2022 meltdown was as tragic as it was historic,” he said. “While it would be easy to kick our company when it’s down, this is our company and consequently our careers and our livelihoods.”

Ahead of the hearing, ABC News obtained messages sent to Southwest Airlines’ cockpits during this winter’s meltdown which illustrate the dysfunction taking place at the company.

“Scheduling is asking to confirm who is operating this flight. Please send Employee ID numbers to confirm. It’s a mess down here,” one message said.

Southwest had no idea where their crews were, who was flying their planes and was unable to contact pilots and flight attendants for days.

The airline flew more than 500 empty flights, many on routes that had been cancelled and could have been full of passengers while more than two million people were stranded.

“No updates here. Scheduling is so far behind we were told we aren’t allowed to walk over and talk to them,” another note from a flight dispatcher to a cockpit read.

Last month, in an exclusive interview with ABC’s Good Morning America, Southwest Airlines CEO Bob Jordan apologized for the debacle.

“There’s just no way almost to apologize enough because we love our customers, we love our people and really impacted their plans,” Jordan said. “There will be a lot of lessons learned that come out of this.”

The chaos cost the company as much as $825 million in lost revenue and added expenses, the company said in a government filing last month.

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