Delta Air Lines CEO Ed Bastian apologized and offered frequent flyer miles to travelers for thousands of flight cancellations as the carrier struggled to recover from Friday’s globe-spanning IT outage, disruptions that sparked criticism from Transportation Secretary Pete Buttigieg.

The Atlanta-based airline canceled more than 4,600 flights from Friday through Sunday, more than any other airline, according to aviation data firm OAG.

On Monday morning, Delta had already canceled another 700 flights, or 18% of its mainline operation, making up the vast majority of cancellations in the U.S.

The delays and cancellations have put Delta in a rare spotlight for the carrier whose leaders pride themselves on reliability and punctuality.

“We continue to receive reports of unacceptable disruptions and customer service conditions at Delta Air Lines, including hundreds of complaints filed with our Department,” Buttigieg said in an emailed statement late Sunday. “I have made clear to Delta that we expect the airline to provide prompt refunds” to customers who chose to call off their trips because of the disruptions as well as “timely reimbursements for food and overnight hotel stays to consumers affected by the delays and cancellations, as well as adequate customer service assistance to all of their passengers.”

The disruptions have persisted at Delta while most other carriers have recovered. American Airlines said it was almost back to normal by Saturday. United Airlines had elevated flight disruptions on Sunday with 9% of its schedule canceled, or 260 flights, according to FlightAware, but still below Delta’s.

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

The airline was offering flight attendants extra pay to pick up shifts, a staff memo on Sunday said. The carrier called some of them on their personal phones to come in, according to a person familiar with the matter. High demand during some one of the busiest periods of summer challenged the airline to find alternative flights for affected travelers, Bastian said in his note.

Delta Air Lines has a number of Microsoft tools that were impacted in the outage, “in particular one of our crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown,” Bastian said in his note.

That would make the event similar to an issue Southwest Airlines suffered, on a much greater scale, at the end of 2022 when it failed to recover from severe winter weather for days.

A botched software update from cybersecurity firm CrowdStrike that paralyzed some Windows-based programs also hit the banking and health-care industries.