Why Delta is being investigated

TThe U.S. Department of Transportation (DOT) has launched an investigation into Delta Air Lines after receiving hundreds of complaints about canceled and delayed flights, which began when the CrowdStrike outage caused technological chaos around the world last week.

U.S. Transportation Secretary Pete Buttigieg announced the news on X on Tuesday, saying the investigation was to “ensure that the airline is complying with the law and taking care of its passengers during the ongoing widespread disruptions.”

The announcement comes after Buttigieg said in a post on X on Sunday that the U.S. Department of Transportation had received hundreds of complaints about the airline for “ongoing disruptions and unacceptable customer service conditions.”

“I have made it clear to Delta that we will hold them to all applicable passenger protections,” Buttigieg said in his Sunday post. “Delta must promptly refund customers who choose not to rebook, offer free rebooking to those who do, and provide timely refunds for meals and hotel stays and appropriate customer service assistance to customers affected by these delays and cancellations.”

“No one should be stranded at an airport overnight or on hold for hours waiting to speak to a customer service representative,” he added.

Read more: Delta apologizes for rough handling of college golf team’s equipment

The CrowdStrike outage, which began Thursday evening after a faulty update was installed on computers running Microsoft Windows, caused thousands of flights to be canceled or delayed worldwide. Since the outage began, Delta has canceled more than 6,600 flights — more than any other airline, according to The Related Press reported.

A Delta spokesperson told TIME that the airline is “fully cooperating with the U.S. Department of Transportation’s investigation.” Delta said on its website that more than half of its IT systems worldwide are Windows-based and that the CrowdStrike outage required the airline’s IT teams to “manually repair and reboot each of the affected systems.”

“We remain fully focused on restoring our operations after cybersecurity vendor CrowdStrike’s faulty Windows update left IT systems around the world inoperable,” the spokesperson said. “Across our operations, Delta teams are working tirelessly to care for customers affected by delays and cancellations and make their lives easier again while we work to restore the reliable, on-time service they expect from Delta.”

Although Delta has been involved in controversial customer service incidents in the past, the airline has often been considered one of the most punctual major carriers. But on Tuesday, Delta had to cancel another 470 flights – even though other airlines such as United and American Airlines appeared to have recovered from the CrowdStrike outage, canceling only 47 and 40 flights respectively, according to FlightAware.