Delta Air Lines Inc. is under investigation by US transportation authorities over its handling of a technology glitch that has led to thousands of canceled flights.

The Department of Transportation opened a probe “to ensure the airline is following the law and taking care of its passengers during continued widespread disruptions,” US Transportation Secretary Pete Buttigieg said Tuesday in a post on X. The flight disruptions prompted 3,000 passengers to file complaints with the federal department, he told reporters in a later briefing.

The decision adds to the strain on the carrier after it said a day earlier that it expects cancellations to stretch later into the week. Delta has fared the worst among US carriers, with most other airlines getting back on track over the weekend after an outage Friday caused by CrowdStrike Holdings Inc. disrupted systems across numerous industries worldwide.

Delta is “fully cooperating” with the investigation and remains “entirely focused” on restoring operations, the airline said in an emailed statement. More than half of its worldwide IT relies on Microsoft Corp.’s Windows operating system affected by the software error.

Airline employees were working to get planes and crews “where they need to be so we can return to normal operations by the end of the week,” John Laughter, operations chief for Delta TechOps, said Tuesday in a note to employees.

The carrier canceled 478 daily flights as of Tuesday evening, or 13% of its normal schedule, according to tracking service FlightAware.com. While that was less than half of Monday’s total, the scrapped departures pushed the tally since Friday’s breakdown to more than 5,400.

Under US rules, disruptions caused by issues within an airline’s control require carriers to issue vouchers for things including meals, bookings on another airline and hotels. Customers are also entitled to a full cash refund if they decline an alternative flight offered after the delay, though they must request the payment.

The widespread cancellations also attracted the attention of key lawmakers in Congress. Legislation signed into law by President Joe Biden in May codified that passengers are entitled to a refund if their flights are canceled or significantly delayed or changed. Senator Maria Cantwell, the Washington Democrat who chairs the Senate panel that oversees aviation, told Delta Chief Executive Officer Ed Bastian in letter that she was concerned the company’s website and policies “do not properly follow this law.”

Online Complaints

Delta customers expressed frustration on social-media platforms, complaining about having their original and rescheduled flights canceled, lost baggage and not receiving food and hotel vouchers.

The carrier’s pilots also weighed in. Darren Hartmann, chairman of the Air Line Pilots Association at Delta, blasted the situation in a letter to members of the union on Tuesday, lamenting the “inability to contact the company in any capacity and the feeling that we have been abandoned in the system” during the disruptions.

The airline abruptly barred anyone under 18 from traveling alone while it’s working through the flight disruptions. Delta implemented the policy on Friday and has since extended it through Tuesday. 

“Those already booked will not be able to travel,” the carrier posted on its website. “Please do not book new travel for unaccompanied minors.”

Delta apologized in a statement for “confusion or frustration” caused by the policy, which was put in place to protect minors from being separated from families or caregivers during flight disruptions.

The company’s longer recovery stemmed from the outage’s impact on an internal system used to process changes to many flights and their crews. That created a cascading effect, leaving Delta unable to get its crews and planes fully aligned.