The U.S. Transportation Department said today it is opening an investigation into Delta Air Lines after the carrier canceled more than 5,000 flights since Friday as it struggles to recover from a global cyber outage that snarled airlines worldwide.

While other carriers have been able to resume normal operations, Delta has continued to cancel hundreds of flights daily of a crew scheduling system.

Since Friday Delta has been canceling 30% or more of its flights daily through Monday, axing 444 flights today, or 12% of its schedule as of 11:00 a.m. and delaying another 590, or 16%, according to FlightAware, after canceling 1,150 on Monday.

Transportation Secretary Pete Buttigieg said today the investigation is to “ensure the airline is following the law and taking care of its passengers during continued widespread disruptions… Our department will leverage the full extent of our investigative and enforcement power to ensure the rights of Delta’s passengers are upheld.”

Delta said it was in receipt of the USDOT notice of investigation and is fully cooperating. “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 airline said. Delta shares were down 0.5% in early trading.

Delta CEO Ed Bastian said Monday it would take the U.S. carrier another couple of days before its operations recover. Since Friday, the carrier had canceled 4,000 flights as of Monday morning.

A software update by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers, including many airlines, on Friday.

Delta is widely respected for running a reliable operation. Analysts say its on-time performance in terms of arrivals and departures has helped the airline cement its position as a premium airline.

The issue has left customers fuming. Many complained they waited hours for assistance as the airline’s helplines were overwhelmed. Some were forced to rent cars, driving hundreds of miles to get to destinations, while others said they would have to wait days for new flights.

In December, Southwest Airlines agreed to a record-setting $140 million civil penalty over the 2022 holiday meltdown that led to 16,900 flight cancellations and stranded 2 million passengers, resolving a USDOT investigation.