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Transportation Secretary spoke to CEO Ed Bastian on Sunday about the airline’s high number of cancellations since Friday. Buttigieg said his agency had received “hundreds of complaints� about Delta, and he expects the airline to provide hotels and meals for travelers who are delayed and to issue quick refunds to customers who don’t want to be rebooked on a later flight.

“No one should be stranded at an airport overnight or stuck on hold for hours waiting to talk to a customer service agent,� Buttigieg said. He vowed to help Delta passengers by enforcing air-travel .

Delta has canceled more than 5,500 flights since the outage started early Friday morning, including at least 700 flights canceled on Monday, according to aviation-data provider Cirium. Delta and its regional affiliates accounted for about two-thirds of all cancellations worldwide Monday, including nearly all the ones in the United States.

United Airlines was the next-worst performer since the onset of the outage, canceling nearly 1,500 flights. United canceled only 17 Monday flights by late morning, however.

Other airlines that were caught up in the first round of groundings also returned mostly to normal operations by Monday. That included American Airlines, Spirit Airlines, Frontier Airlines and Allegiant Air.

Bastian, the Delta CEO, said in a message to customers Sunday that the airline was continuing to restore operations that were disrupted. One of the tools Delta uses to track crews was affected and could not process the high number of changes triggered by the outage.

“The technology issue occurred on the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our re-accommodation capabilities,� Bastian wrote. Loads are the percentage of sold seats on each flight.

Airlines have large, layered technology systems, and crew-tracking programs are often among the oldest systems. When the outage began Friday, it also affected systems used to check in passengers, schedule crews and make pre-flight calculations about aircraft weight and balance, airlines reported. United and American reported intermittent problems communicating with crews in the air, contributing to their decisions to .

Some airlines, including Southwest and Alaska, do not use CrowdStrike, the provider of whose faulty upgrade to Microsoft Windows triggered the outages. Those carriers saw relatively few cancellations. Aviation experts said it was likely that Delta has more systems running on Microsoft Windows than other airlines.

“The impact of the CrowdStrike IT outage will linger on for a few more days yet, and will stay in the minds of travelers with canceled holidays for even longer,� John Grant, senior analyst with travel data provider OAG, said in a blog. “Such events highlight the challenges of an industry dependent on external IT systems that can, and likely will, fail again in the future.�

Atlanta-based Delta has to make it easier for customers to reschedule trips.

Delta's meltdown is reminiscent of the December 2022 debacle that caused Southwest Airlines to cancel nearly 17,000 flights over a 15-day stretch. After a federal investigation of Southwest's compliance with consumer-protection rules, the airline agreed to pay a $35 million fine as part of a with the Transportation Department.

The might be the most visible victim of the worldwide tech problems caused by the faulty software update from Texas-based cybersecurity firm CrowdStrike. Microsoft said the glitch affected 8.5 million machines. CrowdStrike says it has deployed a fix, but experts say it could take days or even weeks to repair .