The collapse at Delta is stunning for an outfit that was widely viewed as the best big US airline.

United States regulators are investigating how Delta Air Lines is treating passengers affected by cancelled and delayed flights as the airline struggles to recover from a global technology outage.

Transportation Secretary Pete Buttigieg announced the Delta investigation on the X social media platform on 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 said.

Delta and its Delta Connection partners cancelled about 500 flights on Tuesday by midday on the East Coast, accounting for about two-thirds of all cancellations in the United States, according to tracking platform FlightAware.

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 cancelled more than 6,600 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. The systems could not keep up with the high number of changes triggered by the outage.

The collapse at Delta is stunning for an outfit that was widely viewed as the best big US airline – the most profitable before and after the pandemic, and the best-run operation. Delta has almost always ranked near the top among all US carriers for on-time performance.

The Department of Transportation 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”.

Senators weigh in

Investigators are likely to focus on whether Delta is complying with federal rules and offering prompt refunds to passengers whose flights are cancelled or significantly delayed. In a text provided to The Associated Press, a Delta passenger whose flight was cancelled on 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 Washington, lawmakers are beginning to weigh in. Senator Maria Cantwell, Democrat from Washington and chair of the Senate committee that oversees airlines, said in a letter to Delta CEO Ed Bastian that she is “concerned” that Delta is not complying with passenger rights contained in a law that Congress passed in May.

“While the technology outage was clearly not caused by Delta or any airline, I am nevertheless concerned that Delta is failing to meet the moment and adequately protect the needs of passengers,” Cantwell wrote.

Delta’s meltdown mirrors that of Southwest Airlines, which cancelled nearly 17,000 flights over 15 days in December 2022. A Transportation Department investigation ended with Southwest agreeing to pay a $35m fine as part of a $140m settlement.

Southwest blamed its breakdown on a winter storm, but other airlines recovered in a couple of days while Southwest did not. Consumer advocates see the same pattern with Delta this month — the airline continues to blame the CrowdStrike outage while rivals such as American recovered quickly. Even United Airlines, the second-worst at cancellations, was back on track Monday.

“It’s not about the thing that caused the problem, it’s about how you recover from the problem. That’s the test of an airline,” said William McGee, a former aircraft dispatcher who is a consumer advocate at the American Economic Liberties Project, a group critical of large corporations.