People wait in line at a Spirit Airlines counter at George Bush Intercontinental Airport on August 5, 2021 in Houston, Texas.

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The causes of Spirit Airlines‘ fixed Flight cancellations that derailed tens of thousands of customers’ summer vacation this week have been brewing for more than a month, said CEO Ted Christie.

A combination of July flight delays, staffing bottlenecks, technology issues and a surge in travel that surprised most airlines culminated in more than 1,700 canceled flights since Sunday, some days of which made up more than half of Spirit’s flight schedule.

And for travelers, the trouble is not over yet. Christie said the airline would have to cancel additional flights in the next few days to get back on track.

“There are definitely some angry people out there,” he told reporters Thursday night. “At the moment I can only say that we are very sorry about what happened.”

The chaos enraged stranded customers at airports and sparked angry news online that presented Christie with one of his biggest tests since he took office as CEO on New Year’s Day 2019.

“That’s a punch in the heart for everyone,” said Christie.

Chronic delays in July piled up, causing staff shortages as crews timed out and reached the maximum time they could legally work each day, he said. It got dramatically worse over the weekend and the days that followed.

“We couldn’t stand in front of it,” said Christie. He estimated “tens or hundreds of thousands” of customers were affected by the disruption at Spirit and said it was too early to assess the financial impact on the company.

On Thursday alone, 446 Spirit flights were canceled, 56% of operations.

A sharp recovery in summer travel has been a headache for summer travelers as airlines and their contractors have faced staff shortages associated with the usual annoying summer storms.

Spirit, based in Miramar, Florida, has improved its reliability over the past few years, becoming in 2017 Walt DisneyThe company’s leadership and training subsidiary, the Disney Institute, to improve customer service.

“We will do everything we can to regain the trust of our guests and the traveling public. We believe we can do that,” Christie said on the call on Thursday. He said the airline is giving cash refunds to affected customers.

In hindsight, Spirit should have canceled more flights earlier to allow time for recalibration, Christie said. Instead, the airline tried to maintain flights to serve large numbers of customers, many of whom were flying for the first time since the pandemic began.

The airline forecast last month that it would fly nearly 11% more flights in the third quarter than in the same period in 2019, a much stronger rebound than most airlines.

Christie said he and other executives are looking at ways to deploy more backup staff, faster response to disruptions, and better technology.

“We’re starting to turn the corner and get our legs under us so we can start going back to where we were before,” Christie said.

Correction: This story has been updated to correct a headline.