Delta Air Lines Inc. apologized for canceling thousands of flights during the busiest travel weekend of the summer as many of its systems failed following the catastrophic CrowdStrike Holdings Inc. IT outage on Friday.

The Atlanta-based carrier stood out as the airline worst affected by the technology meltdown, with US carriers by far the hardest hit over the weekend. The issues continued on Monday, with Delta canceling more than 600 flights, or about 16% of its domestic schedule, according to flight-tracking website FlightAware.

Delta Chief Executive Officer Ed Bastian blamed the impact on a “significant number” of functions that rely on Microsoft Corp.’s Windows operating system, including its crew-tracking tools, rendering the airline unable to process large volumes of changes to flight personnel.

“I want to apologize to every one of you who have been impacted by these events,” Bastian said in a statement, adding, “we understand how difficult it can be when your travels are disrupted.”

All-in, the chaos led to more than 3,500 flights being canceled through Saturday, Bastian said, with more cancellations rippling through as the airline struggled to regain its footing. The disruption occurring during the profitable peak summer period hurt Delta, with flights exceeding 90% capacity, making it harder to rebook customers.

Aviation analytics firm Cirium showed Delta canceling 1,058 flights as of 6 p.m. New York time on Sunday, or 21.4% of its daily schedule. That accounted for the bulk of the almost 1,500 services among US carriers axed nationwide. Among Delta’s peers, United Airlines Holdings Inc. also canceled several hundred flights. 

American Airlines Group Inc., which was among the first companies last Friday to quickly get back to normal, canceled fewer than 100 on Sunday.

As of Monday, FlightAware showed about 50 cancellations combined for American and United as their operations came back on track. 

US Secretary of Transportation Pete Buttigieg criticized Delta’s handling of the chaos, citing its “continued disruptions and unacceptable customer service conditions.”

“Delta must provide prompt refunds to consumers who choose not to take rebooking, free rebooking for those who do, and timely reimbursements for food and hotel stays to consumers affected by these delays and cancellations,” tweeted Buttigieg on X.

Since it erupted Friday, the global travel chaos has affected airlines and airports to varying degrees depending on the systems impacted and the vendors used.

Some self-check in airport systems were rendered useless, passengers couldn’t access ticket reservations, or airlines were not able to use cockpit communications systems. At some airports, airlines resorted to checking in passengers manually with hand-written boarding passes.

A faulty software update by cybersecurity firm CrowdStrike was responsible for 8.5 million computers worldwide seizing up, affecting everything from medical appointments to investment banks and train operators.

After Microsoft, CrowdStrike is the second-biggest maker of “modern endpoint protection” software and controls 18% of the $12.6 billion market, according to research firm IDC.