CrowdStrike has taken a strike back against Delta Air Lines, after the carrier said it plans to pursue legal claims to recover its losses from a huge technology outage last month.

Atlanta-based Delta last week said it has hired law firm Boies Schiller Flexner to pursue claims against software firms Microsoft and CrowdStrike. Delta CEO Ed Bastian said the outage cost the airline $500 million in lost revenue and compensation for passengers.

“You can’t come into a mission critical 24/7 operation and tell us we have a bug,” Bastian said during an appearance on CNBC.

Then on Sunday evening, an attorney for CrowdStrike sent a letter to an attorney for Delta saying the airline’s public threat of litigation “has contributed to a misleading narrative that CrowdStrike is responsible for Delta’s IT decisions and response to the outage.”

Michael Carlinsky, an attorney with the firm Quinn Emanuel who represents CrowdStrike, wrote in the letter to Boies Schiller Flexner attorney David Boies that CrowdStrike “reiterates its apology to Delta, its employees, and its customers, and is empathetic to the circumstances they faced.”

The public spat comes as both CrowdStrike and Delta are struggling to repair reputations battered by the outage.

The CrowdStrike outage affected companies that use Microsoft systems around the world on the morning of July 19. For Delta in particular, the outage triggered problems that the airline struggled to recover from, leading to a massive operational meltdown lasting five days with more than 6,000 flight cancellations.

Thousands of passengers were stranded and stuck in airports during the mass cancellations. The U.S. Department of Transportation is investigating Delta and its treatment of customers.

Between July 19 and Friday’s close of trading, Delta shares dipped about 12%, while CrowdStrike’s stock price was down about 29%, though markets have also raised fears of a broader economic slowdown.

Carlinsky wrote in his letter that CrowdStrike “strongly rejects any allegation that it was grossly negligent or committed willful misconduct,” and added: “Your suggestion that CrowdStrike failed to do testing and validation is contradicted by the very information on which you rely from CrowdStrike’s Preliminary Post Incident Review.”

He also wrote that CrowdStrike “reached out to Delta to offer assistance” and added: “To this day, CrowdStrike continues to work closely and professionally with the Delta information security team.”

Carlinsky pointed a finger at Delta, raising questions about the resilience of Delta’s IT infrastructure and its decisions on system upgrades, as well as why Delta’s competitors “all restored operations much faster” and why Delta turned down help from CrowdStrike professionals “who assisted many other customers to restore operations much more quickly than Delta.”

The letter also emphasized that CrowdStrike’s liability is “contractually capped at an amount in the single-digit millions.”

Since Delta threatened to pursue legal claims, CrowdStrike also demanded Delta preserve all documents, with Carlinsky adding that “while litigation would be unfortunate, CrowdStrike will respond aggressively, if forced to do so, in order to protect its shareholders, employees, and other stakeholders.”

“CrowdStrike hopes Delta reconsiders its approach and agrees to work cooperatively with CrowdStrike going forward,” he added.

The dispute also pits two high-profile law firms against each other. Quinn Emanuel is a major business litigation firm that has represented Elon Musk and Tesla, as well as OpenAI. Carlinsky is co-managing partner of the firm and the global head of complex litigation.

Boies Schiller Flexner has represented clients ranging from victims of Jeffrey Epstein to the U.S. Department of Justice in its antitrust case against Microsoft. David Boies is co-founder of the firm and a well-known litigator. His firm is also known for representing Harvey Weinstein and failed blood-testing startup Theranos.

CrowdStrike issued a statement Sunday saying: “We have expressed our regret and apologies to all of our customers for this incident and the disruption that resulted.”

“Public posturing about potentially bringing a meritless lawsuit against CrowdStrike as a longtime partner is not constructive to any party,” CrowdStrike said in its statement. “We hope that Delta will agree to work cooperatively to find a resolution.”