Hundreds of US flights were canceled Monday as carriers, particularly Delta Air Lines, work to recover four days after a global tech outage caused massive delays and left travelers stranded at airports around the nation.

Delta is facing mounting pressure to get passengers back in the air, as it continues to rack up the majority of canceled US flights. US Transportation Secretary Pete Buttigieg has expressed frustration with the pace of Delta’s recovery and its CEO has apologized to affected travelers.

About 980 flights into, within or out of the United States were canceled by Monday afternoon, and 4,400 flights were delayed, according to flight tracking site FlightAware. Delta passengers have been slammed by more than 4,500 cancellations since Friday, including 789 on Monday, by far the most of any airline.

Delta declined to comment on the numbers, but in an update to customers Sunday afternoon, CEO Ed Bastian said the company is working to restore operations for its customers.

Three of those customers, Nicole DaSilva, Tiffany Denlinger and Melissa Levine were stranded at Hartsfield-Jackson Atlanta International Airport on Monday, surrounded by bags and lines of weary travelers – with no clear way to get home.

Fresh off their vacation in Athens, Greece, the trio has been trying to get back to Arizona for two days. So far, all they have is a stack of canceled flight tickets.

Denlinger has an electronic tracker in her luggage and said it’s bounced from concourse to concourse at the Atlanta airport. But because she can’t access her suitcase, she hasn’t been able to use her laptop – leading to lost commissions and income.

While Delta says it plans to cover hotel, meal or ground transportation for affected customers, the airline’s policy says it does not compensate passengers for lost income due to canceled flights.

DaSilva and Levine managed to find extended care for their dog back in Tucson only to have the air conditioning go out. Tucson is expected to hit a high of 104 degrees Monday. Now the couple is scrambling to find care for their dog somewhere out of the extreme heat.

Denlinger said she believes the airline was not prepared for this situation, “and we’re just kind of the guinea pigs to something truly catastrophic.”

The “largest IT outage in history” – prompted by a software update for Microsoft devices late Thursday into the early hours of Friday – impacted an estimated 8.5 million Windows devices worldwide, causing disorder at airports, outages for 911 services and challenges at health care facilities.

While the CrowdStrike problem affected computers meant to check customers in for many airlines, it also disrupted Delta’s crucial crew scheduling software.

“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 the statement.

It didn’t help that the outage took place on what Bastian described as “the busiest travel weekend of the summer,” with 90% of the airline’s flights booked.

Here’s what we know about the outage and its continued effects.

Outage caused by a CrowdStrike update

The company said the outage was not caused by a security incident or a cyberattack, but rather a software defect.

The issue was identified and isolated, and engineers deployed an update to fix the problem, CrowdStrike CEO George Kurtz said.

Kurtz has apologized to customers and said the company is “deeply sorry for the inconvenience and disruption.”

In an update posted to LinkedIn late Sunday, CrowdStrike said “a significant number” of the 8.5 million devices were back online and operational.

Overall costs from the outage could top $1 billion, Patrick Anderson, CEO of Anderson Economic Group, told CNN.

Chaos and confusion for travelers

On Sunday, more than 1,800 US flights were canceled and more than 9,900 delayed, according to FlightAware. Major hotels, including Marriott International and some Hiltons, were impacted both in regard to payment processing and delays to check in-processes.

US-based carriers American Airlines, United Airlines, Delta Air Lines, Allegiant Air, Sun Country and Frontier Airlines all reported issues on Friday.

Delta paused all its flights Friday morning while its systems were offline, and more than 3,500 Delta and Delta Connection flights were canceled through Saturday, according to the airline. More than 1,200 Delta flights were canceled on Sunday, after the carrier canceled 1,208 flights on Saturday, according to FlightAware.

Delta said it would not be allowing unaccompanied minors to travel until July 24, due to the outage.

Experts are urging travelers who have been forced to cancel trips to look into refund options. New federal rules say travelers deserve cash refunds when inconvenienced by their airline – not vouchers or travel credits.

Buttigieg spoke with Delta’s Bastian on Sunday afternoon as the air carrier continues to struggle with a high number of flight cancellations, according to the department.

“I have made clear to Delta that we expect the airline to provide prompt refunds to consumers who choose not to be rebooked, and free rebooking and 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,” Buttigieg said in a statement.

Other airlines largely got back on track over the weekend, including one of the world’s largest operators, Turkish Airlines, which said Saturday, “The global technical issue has been resolved. All our operations have returned to normal.”

The same day, Jetstar Japan, Hong Kong Express and Cebu Pacific airlines said their operations are gradually being restored after disruptions due to the global tech outage.

Even with the flawed computer update rolled back, it’s not a quick fix for airlines, which have computers at thousands of gates that will need to be individually rebooted, David Kennedy, cofounder of cybersecurity company Binary Defense, told CNN on Saturday.

“It’s not just as simple as rebooting. There’s a lot more steps and complexities in this that are involved,” Kennedy said. “There’s just not enough people at those airports, at those locations to go and do it.”

Amid the travel chaos, Buttigieg said earlier in a social media post Saturday he received reports of some airlines only offering flight credits to passengers for canceled flights.

“Let me be clear – you are entitled to get your money back promptly if your flight is canceled and you don’t take a rebooking,” Buttigieg said.

Hospitals, 911 systems and government agencies affected

The widespread usage of CrowdStrike’s software meant the outage penetrated numerous industries, including emergency communications, government services and health care.

Large hospital systems, including Mass General Brigham in Massachusetts, Penn Medicine in Pennsylvania, Mount Sinai Health System in New York all reported Friday they were affected by the outage.

Emory Healthcare in Atlanta said it had delayed “procedures at ambulatory surgical centers and hospitals are delayed until our systems are stabilized.”

Some cancer centers, including Dana-Farber Cancer Institute in Boston and New York’s Memorial Sloan Kettering Cancer Center, also paused certain procedures and scheduled appointments.

Several blood banks likewise experienced disruptions.

And in a few areas, including Arizona and Alaska, 911 services were briefly disrupted before being restored.

Government agencies, including Social Security offices and local Department of Motor Vehicles offices, also temporarily paused their operations due to the outage. Public transportation in Washington, DC, and Pennsylvania were affected but restored operations Friday.