The US Department of Transportation has opened an investigation into Delta Airlines following its mass cancellation of flights after a worldwide IT outage.

While most airlines recovered not long after a major CrowdStrike outage took down critical services globally on Friday, July 19, after the weekend Delta was still experiencing disruptions.

By Tuesday, July 23, passengers were still unable to take flight with Delta Airlines. Delta's CEO said the issue would be resolved in a few more days.

Delta and its regional affiliates accounted for about two-thirds of all flight cancellations worldwide, CBS News reported.

In total, Delta was forced to cancel more than 6,000 flights. The company stated that the CrowdStrike failure rendered its IT systems inoperable.

US transportation secretary Pete Buttigieg said that the department was looking into "how it could be that days after the other airlines are back to normal, Delta is still canceling hundreds of flights.”

“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” affected by the disruptions.

According to the airline, more than half of its technology systems are Windows-based, including a key tool used to schedule pilots and flight attendants.

Operations resumed as normal on Thursday, July 25. The airline is refunding customers whose travel was disrupted.

Airlines are heavily impacted by disruptions to their IT equipment or other digital infrastructure. In 2023, the UK's air traffic control manager experienced a network-wide outage. That same year, British Airways had to cancel 175 flights at London Heathrow due to an outage, its second in just six months.

In Germany, Lufthansa flights were grounded by an IT outage in February 2023. Earlier this year, a deliberate cut to an AT&T fiber cable cut led to travel disruption at Sacramento airport.

In June 2024, the Swiss air navigation service Skyguide experienced an outage when thunderstorms caused flooding, which impacted its data center's cooling system.