The technology outage left you stranded when your flight was canceled. Here's what to do.

U.S. airline regulators have opened an investigation into Delta Air Lines, which is still struggling to restore operations on Tuesday, more than four full days after a faulty software update caused technological havoc worldwide and disrupted global air travel.

Transportation Secretary Pete Buttigieg announced the Delta investigation on the X social media platform Tuesday “to ensure the airline is following the law and taking care of its passengers during continued widespread disruptions.”

“All airline passengers have the right to be treated fairly, and I will make sure that right is upheld,” Buttigieg added.

That could include a full refund if your flight was canceled and other compensation depending on where you were traveling.

As of Tuesday morning, Delta had an outsized portion of canceled flights. Of the 679 total cancellations of flights within, into or out of the United States, Delta had 455 of them, according to data posted by FlightAware as of 11:55 a.m. Eastern time Tuesday.

United Airlines was next among U.S. airlines with 45, followed by American with 38.

Delta also had more delayed flights than any other U.S. airline, with 696, but the gap was not as wide: American had 507 flight delays.

The outage began Thursday night into Friday morning, after a faulty software upgrade from cybersecurity company CrowdStrike to more than 8 million Microsoft computers around the world.

The Atlanta-based carrier has canceled more than 7,000 flights since the outage started, far more than any other airline, according to figures from FlightAware and travel-data provider Cirium.

Delta said it was cooperating with the investigation.

“We remain entirely focused on restoring our operation after cybersecurity vendor CrowdStrike’s faulty Windows update rendered IT systems across the globe inoperable,” an airline spokesperson said in a statement. "Across our operation, Delta teams are working tirelessly to care for and make it right for customers impacted by delays and cancellations as we work to restore the reliable, on-time service they have come to expect from Delta.”

Delta has said upward of half its technology systems run on Microsoft Windows, including a tool the airline uses to schedule pilots and flight attendants. That system could not keep up with the high number of changes triggered by the outage.

The Transportation Department said it launched the investigation after seeing Delta's continued widespread flight disruptions “and reports of concerning customer service failures.”

The department said the investigation will evolve as it “processes the high volume of consumer complaints we have already received against Delta.”

Investigators are likely to focus on whether Delta is complying with federal rules and offering prompt refunds to passengers whose flights are canceled or significantly delayed. In a text provided to The Associated Press, a Delta passenger whose flight was canceled Saturday was told, “If you prefer not to rebook your trip, your ticket value will automatically be available as an eCredit that can be used towards a future Delta ticket.”

In an announcement to travelers posted on its website Monday, Delta said that its crews are fully staffed, but that a critical operational system that ensures all flights have a full crew "is deeply complex and is requiring the most time and manual support to synchronize.”

Buttigieg spoke to Delta CEO Ed Bastian on Sunday about the airline’s high number of cancellations. The Transportation Department said its top officials have reminded Delta of the airline’s obligation to provide refunds to passengers whose flights were canceled and who don’t want to be rebooked on a later flight.

Here is what you can do if you are caught up in the cancellations or delays.

Contact your airline

Airlines should rebook passengers automatically, but that could take much longer as carriers recover from the outage, so passengers will have to take more initiative. And be more creative.

“People who are already at the airport usually go up to the help desk, but there’s a line of 500 people. It’s going to take all day,” said Jesse Neugarten, the founder and CEO of travel-search site Dollar Flight Club. He suggested calling the airline and using an international help-desk number, if there is one, to reach an agent more quickly.

Another tactic is to post a few words to the airline on the social platform X. Many airlines have staffers who will help rebook passengers who contact the carrier through social media.

Check other airlines and airports

Use your airline's app: It may have more-current information about flight status than delays and cancellations displayed in the airport terminal.

In the U.S., Southwest and Alaska Airlines said they were not affected by the outage. However, since it's summer vacation season, those airlines have a limited number of available seats left.

Neugarten suggested checking nearby airports for available flights, which might require changing airlines.

That's because many flights are sold out or have just a few empty seats during the peak summer vacation season, which leaves airlines with less room to accommodate passengers from canceled flights.

Can I get a refund if my flight is canceled?

Passengers whose flights are canceled are entitled to a full refund in the form of payment they used to buy the ticket. That’s true even if the ticket was sold as non-refundable.

A refund may be acceptable to travelers who no longer want to make the trip, but many people just want another way to reach their destination, and buying a last-minute replacement ticket could cost more than the refund will cover.

Airports in Amsterdam, Berlin and Zurich had particularly high numbers of canceled flights. When airlines in Europe are responsible for a cancellation or delay, travelers are entitled to compensation under a regulation known as EU261, but that rule has an exception for disruptions caused by "extraordinary circumstances" beyond the airline’s control.

There is no provision for compensation under U.S. law; airlines set their own policies for reimbursing stranded travelers for things like hotels and meals. The Transportation Department maintains a “dashboard” showing what each airline promises to cover.

Am I eligible for reimbursements if my flight is canceled?

Airports in Amsterdam, Berlin and Zurich had particularly high numbers of canceled flights.

When airlines in Europe are responsible for a cancellation or delay, travelers are entitled to compensation under a regulation known as EU261, but that rule has an exception for disruptions caused by "extraordinary circumstances" beyond the airline’s control.

There is no similar provision for compensation under U.S. law, and airlines set their own policies for reimbursing stranded travelers for things like hotels and meals.

However, the Transportation Department appears to be taking the view that the cancellations and delays were within the airlines' control, and it is pressuring carriers to cover passengers' costs.

“We have reminded the airlines of their responsibilities to take care of passengers if they experience major delays,” Buttigieg said.

Trade group Airlines for America said carriers hit by the outage were offering rebooking, vouchers, refunds and other help “during this regrettable situation outside of the airlines’ control.”

The Transportation Department fined Southwest $35 million last year as part of a $140 million settlement to resolve an investigation into nearly 17,000 canceled flights in December 2022.

The department maintains a “dashboard” showing what each airline promises to cover during travel disruptions.