Department of Transportation opens investigation into Delta over flight disruptions

The U.S. Department of Transportation is opening an investigation into Delta Airlines over recent flight disruptions, Transportation Secretary Pete Buttigieg said on Tuesday in a post on X.

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

The airline is struggling to resume normal service five days after a global IT outage at cybersecurity firm CrowdStrike.

Delta canceled more than 400 flights on Tuesday morning, the most of any major airline, according to flight tracking site FlightAware. The airline with the second-most cancellations on Tuesday has nixed only 55 flights, FlightAware says.

The outage at CrowdStrike on Friday hindered services at airlines and hospitals in the U.S., banks in Europe and a media company in Canada.

Thousands of flights were canceled across a host of airlines on Friday, but many companies have limited disruptions over the days since.

In a statement to ABC News on Tuesday, Delta said it is fully 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. 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," the company said.

The outage, which affected CrowdStrike clients that use Windows operating systems, disrupted a critical system that ensures each flight has a full crew, the company said on Monday.

"Upward of half of Delta’s IT systems worldwide are Windows based. The CrowdStrike error required Delta’s IT teams to manually repair and reboot each of the affected systems, with additional time then needed for applications to synchronize and start communicating with each other," the company said.

Buttigieg requested that Delta passengers share information with the Department of Transportation.

"While you should first try to resolve issues directly with the airline, we want to hear from passengers who believe that Delta has not complied with USDOT-enforced passenger protection requirements during the recent travel disruptions," Buttigieg said.

"We will follow up," he added.

Ayesha Ali contributed to this report.