Delta Air Lines CEO Ed Bastian on Friday offered employees two free travel passes to thank staff members who were caught in massive disruptions last month sparked by a botched CrowdStrike software update that stranded thousands of customers and crew.

Delta had more trouble than competitors in recovering from the outages that took thousands of Windows machines offline. The carrier canceled more than 5,000 flights from July 19 through July 24, more than it did in all of 2019, according to FlightAware, in an incident that CEO Bastian said earlier this week cost the company about $500 million, a sum that is equal to about 40% of Delta’s second-quarter profit.

The disruption “has been a humbling moment for our company,” Bastian said in his note on Friday, which was seen by CNBC. “I know it’s been extremely difficult, and I’m deeply sorry for what you have endured. An operational disruption of this length and magnitude is simply unacceptable — you and our customers deserve better.”

The flight cancellations and delays stranded thousands and scarred Delta’s reputation as a standout in reliability. Its executives frequently point out Delta’s work to win over customers willing to pay more to fly the carrier, marketing itself as a premium airline.

Bastian said Delta plans to pursue legal action against CrowdStrike and Microsoft “to recover our losses caused by the outage” and that it has hired law firm Boies Schiller Flexner.

“Your efforts throughout have been nothing short of heroic,” he told staff.

Microsoft and CrowdStrike didn’t immediately comment on Friday.