Delta Air Lines struggled for a fourth straight day to recover from a worldwide technology outage caused by a faulty software update, stranding tens of thousands of passengers and drawing unwanted attention from the federal government.

The airline's chief executive said it would take "another couple days" before "the worst is clearly behind us." Delta's chief information officer said Monday that the airline was still trying to fix a vital crew-scheduling program.

Other carriers were returning to nearly normal levels of service disruptions, intensifying the glare on Delta's relatively weaker response to the outage that hit airlines, hospitals and businesses around the world.

Transportation Secretary Pete Buttigieg spoke to Delta 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 consumer-protection rules.

Bastian said in a video for employees that he told Buttigieg, "You do not need to remind me. I know, because we do our very best, particularly in tough times, taking care of our customers."

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 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 briefly ground all flights.

Some airlines, including Southwest and Alaska, do not use CrowdStrike, the provider of cybersecurity software whose faulty upgrade to Microsoft Windows triggered the outages. Those carriers saw relatively few cancellations.

Delta, however, said that "upward of half" its IT systems are Windows-based. The airline said the outage forced IT employees to manually repair and reboot each affected system and synchronize applications so they start working together.

"It is going to take another couple of days before we are in a position to say that ... the worst is clearly behind us," Bastian told employees Monday. "Today will be a better day than yesterday, and hopefully Tuesday and Wednesday will be that much better again."

On the same video, Delta Chief Information Officer Rahul Samant said two applications were particularly difficult to restart on Friday: One that manages traffic at Hartsfield–Jackson Atlanta International Airport, Delta's biggest hub, and another that assigns pilots and flight attendants to flights.

Technicians had gotten the crew-scheduling program running, "but we have a catch-up to do," and new issues keep arising, Samant said.

Atlanta-based Delta said it is offering waivers to make it easier for customers to reschedule trips.

That was of little help to Jason Helmes, a fitness coach who was trying to get home to Detroit from Denver. His flight on Sunday was delayed three times before it was canceled; by the time the plane finally pushed back from the gate, the pilots were at the end of their legally allowed shift.

"Everyone was just stranded. No information on hotels. No information on what to do next," Helmes said. "They said, 'Go down to the luggage carousel, your luggage should be there.' There were thousands of bags down there. I found my luggage — I got lucky."

Helmes said Delta offered to rebook him on Wednesday, but he worried that flight would also be canceled. He booked a Tuesday flight home on Frontier Airlines — one of the carriers that has largely recovered. He is saving his receipts, including a hotel room, in hopes that Delta will reimburse him.

"For the last 10 years, I've been exclusively on Delta," he said. "This has me double-thinking about that."

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 $140 million settlement with the Transportation Department.

Southwest's breakdown started during a winter storm, but the airline's recovery took unusually long because of problems with a crew-scheduling system -- a striking similarity to Delta's current mess.

The airline industry 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 every affected computer.