Delta Air Lines canceled hundreds of flights today, hours after Secretary of Transportation Pete Buttigieg singled out the airline as it struggles to recover three days after a global software outage grounded flights around the world.

Buttigieg said on Sunday that his office had received complaints about Delta’s customer service, and warned that the carrier must provide its customers with adequate assistance and refunds. Delta canceled about 1,300 flights on Sunday, roughly the same number as each of the previous two days, and delayed another 1,600, according to flight tracking website FlightAware. The cancellations represented about a third of its scheduled flights.

Delta’s cancellations today — 650 as of 9 a.m. ET (3 a.m. Hawaii time) — accounted for about 17% of its scheduled departures.

The tech outage on Friday hit airlines especially hard. A flawed update from CrowdStrike, whose software is used around the world, forced Allegiant Air, American Airlines, Spirit Airlines and United Airlines to ground flights, according to the Federal Aviation Administration.

Delta has been the slowest to restore its operations. It canceled about 1,200 flights each on Friday and Saturday, according to FlightAware, while cancellations for other airlines moderated into the hundreds or dozens.

In a statement on Sunday, Delta CEO Ed Bastian apologized to customers. “Canceling a flight is always our last resort, and something we don’t take lightly,” he said.

The outage, which disrupted devices that run Microsoft Windows software, had affected one of Delta’s crew tracking-related tools, Bastian said.

Software that helps airlines schedule and track their pilots and flight attendants can become an Achilles’ heel for companies trying to recover from major disruptions to their operations.

A few days before Christmas in 2022, bad weather severely hampered several U.S. carriers but the problems were most acute at Southwest Airlines, which lacked the equipment it needed to cope with the weather. In addition, a system the company used to match crews to flights could not keep up with a very large number of changes, forcing the company, at one point, to manually reschedule pilots and flight attendants.

Southwest, one of the largest U.S. airlines, canceled nearly 17,000 flights — more than one-third of those scheduled — over the last 10 days of that year. That episode cost the company more than $1 billion and severely damaged its longstanding reputation for efficiency and on-time performance.

Delta appears to be faring better than Southwest, but its elevated cancellations and delays three days after the technology outage raises questions about why the airline has struggled so much more than other companies to recover from the technology outage. Delta has marketed itself as a premium airline, and it had widely been considered by Wall Street analysts as one of the most profitable and best run company in the industry.

The company said it had offered travel waivers to all customers booked on flights between Friday and Sunday, allowing them to change their flights once without cost. Delta said it was offering meal vouchers, hotel accommodation and transportation where available.

But some passengers said that Delta did not provide them hotel rooms when their flights were canceled, leaving people to make their own arrangements or sleep at airport terminals.

Buttigieg said in a social media post on Sunday night that the Department of Transportation had received hundreds of complaints about the airline. Delta must provide prompt refunds to customers who do not want rebooked flights, and timely reimbursements for food and hotel stays for those affected by the delays, he said.

“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, adding that customers should report airlines that fail to honor their customer service requirements to the agency.

Airlines initially treated the outage as something that was inherently outside their control, for which their only obligation to passengers was free rebookings. However, the Transportation Department said on Friday that software outage was considered within airlines’ control, and U.S.-based airlines must provide affected customers compensation for flight disruptions.

This article originally appeared in The New York Times.