The U.S. Department of Transportation (DOT) has opened an investigation into Delta Air Lines, after receiving hundreds of complaints over canceled and delayed flights that started when the CrowdStrike outage last week caused technological chaos worldwide.

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

The announcement comes after Buttigieg said in a post on X on Sunday that the DOT received hundreds of complaints about the airline over “continued disruptions and unacceptable customer service conditions.”

“I have made clear to Delta that we will hold them to all applicable passenger protections,” Buttigieg said in Sunday’s post. “Delta must provide prompt refunds to consumers who choose not to take rebooking, free rebooking for those who do, and timely reimbursements for food and hotel stays to consumers affected by these delays and cancellations, as well as adequate customer service assistance.”

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

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The CrowdStrike outage, which began Thursday night after a faulty update was deployed to computers running Microsoft Windows, caused thousands of flights to be canceled or delayed across the world. Since the outage started, Delta has canceled more than 6,600 flights—more than any other airline, The Associated Press reported.

A Delta spokesperson told TIME in a statement that the airline “is fully cooperating” with the DOT’s investigation. Delta said on its website that more than half of its IT systems worldwide are Windows-based, and that the CrowdStrike outage meant that the airline’s IT teams had to “manually repair and reboot each of the affected systems.”

“We remain entirely focused on restoring our operation after cybersecurity vendor CrowdStrike’s faulty Windows update rendered IT systems across the globe inoperable,” the spokesperson said. “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.”

While Delta has been embroiled in controversial customer service incidents in the past, the airline has often ranked as one of the most on-time large airlines. But on Tuesday, Delta had another 470 flight cancellations—even as other airlines, like United and American Airlines, had seemingly recovered from the CrowdStrike outage with just 47 and 40 flight cancellations, respectively, according to FlightAware.