Delta was the hardest-hit US airline during the mass IT outage, and 3 days later, it's still suffering

• A CrowdStrike update on Friday caused a global IT outage, disrupting flights, trains, and banks.
• Delta Air Lines was particularly badly impacted, canceling over 4,500 flights in four days, per FlightAware.
• Delta's CEO apologized, citing issues with Microsoft Windows affecting crew-tracking tools.

A mass IT outage connected to a flawed update issued by CrowdStrike , a cybersecurity firm based in Austin, disrupted flights, trains, banks, and media outlets worldwide.

Airlines were hit especially hard, with United Airlines , American Airlines , Spirit Airlines, and others grounding flights and announcing delays.

Delta Air Lines, however, was the worst affected by flight cancellations on Friday, the day of the incident, and in the subsequent days.

Delta canceled a total of 1,207 flights on Friday, almost a third of its total flights, according to FlightAware , a flight tracking data platform.

The airline with the next highest number of cancellations was United, which canceled 694 flights.

Over the weekend, most airlines managed to return to some semblance of normality, but three days after the outage, Delta is still struggling to restore operations to full capacity.

On Saturday, it canceled 1,208 flights, and on Sunday it canceled 1,384. This represented 36% of its scheduled departures on both days.

There were 709 cancellations for Delta on Monday as of 11 a.m. ET, making up 19% of Delta flights. There were 1,335 flight cancellations in total across airlines around the world.

Over the weekend, over 40% of Delta flights were delayed, FlightAware reported.

In total, Delta has canceled more than 4,500 flights since Friday, FlightAware's data shows.

By contrast, on Sunday, the worst affected airline after Delta, United, canceled 266 flights, with cancellations for most others in the tens or single digits. On Monday, next to Delta, China Eastern Airlines canceled 109 flights by 11 a.m. ET, representing just 4% of scheduled departures.

Delta's CEO says sorry

In a statement on Sunday, Delta's CEO Ed Bastian apologized to every customer whose travel plans had been disrupted by the outage.

"The technology issue occurred on the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our reaccommodation capabilities," he said.

Bastian said many of Delta's applications use the Microsoft Windows operating system, which was affected by the meltdown. One of the airline's crew-tracking-related tools had become unable to effectively process the number of changes prompted by the software shutting down.

He announced that customers who experienced flight disruptions were being offered meal vouchers, hotel accommodations, and transportation where possible, along with Delta SkyMiles , the airline's loyalty program that offers points to passengers for upgrades and flights, and travel vouchers.

Delta did not immediately respond to a request from Business Insider for further comment.