Airline companies continue to struggle to fully restore operations after the largest information technology outage in history halted global operations on Friday.

A faulty computer software update from cybersecurity company CrowdStrike caused Microsoft cloud computing services to shut down, which affected airlines, banks, hospitals, train stations, media outlets, and even 911 services in some U.S. states, as previously reported by The Dallas Express.

Data from FlightAware shows that from Friday through Sunday, 11,015 flights worldwide were canceled, and 115,860 were delayed. As of 1 p.m. on Monday, 1,539 flights had been canceled, and 24,952 had been delayed.

Delta Airlines tops the list for the most canceled flights over the last three days, with 3,382.

Delta CEO Ed Bastian issued a public apology, explaining how the technology outage affected the carrier’s operations.

“Canceling a flight is always a last resort, and something we don’t take lightly,” Bastian said in a news release. “The technology issue occurred on the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our reaccommodation capabilities.”

“I want to apologize to every one of you who have been impacted by these events. Delta is in the business of connecting the world, and we understand how difficult it can be when your travels are disrupted,” Bastian said.

Bastian also elaborated that the system outage affected one of the company’s crew tracking-related tools, further contributing to cancellations and delays.

Pete Buttigieg, U.S. Transportation Secretary, said he made clear to Delta that the federal administration expects the airline company to issue refunds to passengers who have been affected by the outage and who do not want to be rebooked on another flight, WFAA reported.

Delta has issued a travel waiver that will allow passengers to make a one-time change to their itinerary. Fare differences will be waived for travel that is rebooked on or before July 28, according to the Delta advisory.

Several other major airlines are also still struggling to restore operations, including Spirit and United, per CBS News Texas.