As a winter storm bore down on a broad swath of the U.S., a staffing crisis was brewing for Southwest Airlines in Denver.

Chris Johnson, the carrier’s vice president of ground operations, declared a “state of operational emergency” at the airport after “an unusually high number” of employee absences, according to a Dec. 21 message to ramp workers seen by Bloomberg News.

It was just the beginning.

The so-called bomb cyclone kicked off a cascade of disruptions that have battered Southwest’s operations over the past week, forcing the carrier to cancel thousands of flights and stranding holiday travelers who now face days of waiting.

The chaos is still unfolding. As of Wednesday morning, Southwest had scrapped more than 60% of its Wednesday schedule – more than 2,500 flights – plus 58% of its trips for Thursday, according to FlightAware data.

And although no airline was spared the storm’s wrath, rivals such as Delta Air Lines, American Airlines Group and United Airlines Holdings largely returned operations to normal this week.

Chief Executive Officer Bob Jordan apologized again late Tuesday in a video posted by Southwest, saying the company reduced its schedule to gain time to reposition aircraft and crews.

“We’re optimistic to be back on track before next week,” Jordan said. As of early Wednesday, Southwest has canceled only about two dozen flights for Friday.

Southwest shares were down about 1.2% in premarket trading Wednesday after falling 6% on Tuesday, the most since July, to extend their 2022 decline to 21%. U.S. authorities and lawmakers, meanwhile, are scrutinizing the carrier’s response to the storm, which analysts at Citigroup estimate could shave as much as 5% from Southwest’s fourth-quarter profit.

Customers complained on social media of spending hours in line or on hold to book alternate flights, only to find few alternatives.

Pilots and flight attendants, meanwhile, faced lengthy waits for work assignments and hotel accommodations as the storm and its subsequent disruption hobbled the carrier’s crew scheduling systems and left the company’s fleet of Boeing 737s out of position across the country.

Management messages to Southwest employees seen by Bloomberg News highlight how the chaos unfolded, and how the carrier struggled as the systems used to coordinate a vast network of airplanes, destinations and flight crews were first overwhelmed and then failed to recover.

The airline confirmed the authenticity of the memos and in previous statements has apologized to customers and tried to explain what led to the crisis.

“Part of what we’re suffering is a lack of tools,” Jordan, a 34-year Southwest veteran, said to employees late on Christmas Day. The company’s crew scheduling system is one area in need of investment, he said.

“We need to be able to produce solutions faster. We need to be able to communicate with each other where it doesn’t involve a phone call,” Jordan said.

The effects of the storm were building well before Christmas.

By the evening of Dec. 21, frigid temperatures, high winds and heavy precipitation were disrupting Southwest operations in Denver, limiting the number of flights the carrier could handle safely, Chief Operating Officer Andrew Watterson said in one message.

The carrier suspended operations at Chicago’s Midway airport, meaning both locations had fewer flights shuttling crews between their home bases and assignments, he said.

By Dec. 23, 90% of Southwest’s routes had been affected by weather. Yet, Watterson said, actions to deal with the chaos had positioned the carrier for fewer cancellations on Saturday, Christmas Eve.

“As long as there’s not another disruption, we’ll start the day in a much better position,” Watterson said.

Yet by late Dec. 23, Southwest was “heavily disrupted, undoing all of the work” to position crew members, Watterson said in an update the next day.

Unexpected fog triggered a ground stop and delays in San Diego. A glitch slowed refueling in Denver.

Southwest’s Dallas Love Field stronghold was packed with parked aircraft to ease congestion at northern airports battered by cold, without enough gates to accommodate them.

Many flight crews began Dec. 24 either out of position or resting under U.S. aviation safety rules, leaving the airline with “no choice but to implement additional cancellations,” Watterson said.

By Christmas Day, hundreds of Southwest flights were awaiting crew assignments even as improving weather helped alleviate a majority of its ground operational woes and competitors were restoring their schedules.

Supporting crew scheduling had become the company’s “larger focus,” with teams working extra shifts to troubleshoot problems with an “all-hands on deck” approach, Watterson said in a Dec. 25 update.

The airline is in the process of upgrading its crew systems. The work by teams struggling to overcome the crew and scheduling chaos is “highly manual” and can’t be addressed with more people alone.

“Our current systems are overmatched in situations of this scale,” Watterson said.

In his video Tuesday night, Jordan said the company would double down on plans to upgrade systems “so that we never again face what’s happening right now.”

Once Southwest gets its planes back in the air, it will have to answer questions from lawmakers and U.S. transportation officials.

Jordan spoke Tuesday with Transportation Secretary Pete Buttigieg, who conveyed the Department’s expectation that Southwest meet its obligations to passengers and workers and take steps to prevent a situation like this from happening again.

“This has clearly crossed the line from what’s an uncontrollable weather situation to something that is the airline’s direct responsibility,” Buttigieg said Tuesday on the NBC Nightly News.

Senate Commerce Committee Chair Maria Cantwell, a Democrat from Washington, said her panel will be investigating, while President Joe Biden said in a tweet that his administration is working to hold airlines accountable.

“The problems at Southwest Airlines over the last several days go beyond weather,” Cantwell said in a statement. “The committee will be looking into the causes of these disruptions and its impact to consumers.”

She also used the meltdown to buttress calls to strengthen rules on compensating customers.

“Consumers deserve strong protections, including an updated consumer refund rule,” she said.

Two other senators, Democrats Ed Markey of Massachusetts and Richard Blumenthal of Connecticut, said the airline should compensate customers, not just for the cost of their tickets and accommodations but for ruined holidays as well.