Delta Air Lines is up and running nearly a week since CrowdStrike's global outage rippled through every part of the economy and temporarily halted the airline industry. 

However, while rivals were able to swiftly get back up and running within a day or two, problems at Delta got worse, Transportation Secretary Pete Buttigieg told FOX Business' Neil Cavuto on "Cavuto: Coast to Coast" Wednesday. 

Delta has had roughly 5,472 cancellations and 8,215 delays since the outage occurred Friday, according to data from flight-tracking website FlightAware. Its regional carrier Endeavor canceled 800 flights and delayed over 1,100.

While Delta said its operations would be back to normal Thursday, this issue raised concerns about the carrier's operation and customer support service, prompting an investigation by the Transportation Department. 

DOT LAUNCHES INVESTIGATION INTO DELTA AMID ONGOING FLIGHT DISRUPTIONS

"So, every other airline got back on its feet within a day or two. Delta actually seemed to move in the opposite direction, which is why we are paying special attention," Buttigieg said. 

The department is focusing on why the carrier had so many disruptions and issues related to customer service. 

GLOBAL TECHNOLOGY OUTAGE DISRUPTS MAJOR AIRLINES, 911 SERVICES AND BUSINESSES

The agency has heard reports of people waiting eight hours to get someone on the phone, Buttigieg said during Wednesday's interview. 

"On hold for hours, sleeping on airport floors, unaccompanied minors stranded—these are the stories we are hearing from Delta passengers," Buttigieg posted on X. 

Delta Chief Information Officer Rahul Samant said 60% of the airline's "most critical applications that run the airline" are on Microsoft Windows. 

When the outage occurred, "all of them were rendered inoperable early on Friday," Samant said in a video update to customers earlier this week. 

Samant said the company was notified about the outage around 1:15 a.m. By 3:30 a.m, it figured out a way to "manually touch each one of those to get the bug out and get them back on their feet," according to Samant. That process took a few hours, he said. 

Around 7:30 a.m., the carrier was able to push back its first flight in Atlanta. However, he noted that while applications started coming back in the early hours of Friday, there was a lot of pent-up data that needed to sync across all the applications, which took several hours. 

One of its key applications, though, took much longer to get back up and running.

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"We have two applications that are key, one that allows our Atlanta tower to really holistically manage our biggest hub in terms of gating arrivals, departures. And that application is very data hungry. It took an extra couple of hours to come up, which meant we had a very sluggish start in Atlanta," Samant said.

CEO Ed Bastian reiterated the gravity of the issue, saying the bug forced the company to pause its entire operation and technology platform and that resetting everything "created a number of startup issues on a couple of very, very important applications." 

Bastian said in a note to customers that he understands that passengers were frustrated "with the pace of progress." 

Bastian apologized to affected customers and promised them either SkyMiles Program miles or a travel voucher to make up for the incident.