On Tuesday, Microsoft shot back at Delta after the airline publicly revealed on Monday it would seek damages from the tech giant and CrowdStrike.

Delta attorney David Boies contends there is “reason to believe” that Microsoft “failed to comply with contractual requirements and otherwise acted in a grossly negligent, indeed willful, manner in connection with the Faulty [CrowdStrike] Update” that caused Windows systems to crash, he wrote in a July 29 letter to Microsoft’s chief legal officer, Hossein Nowbar.

In response, a Microsoft representative expressed empathy for customers affected by the CrowdStrike incident but was critical of Boies’ assessment.

“But your letter and Delta’s public comments are incomplete, false, misleading and damaging to Microsoft and its reputation,” Mark Cheffo, co-chair of Dechert’s global litigation practice, wrote Tuesday to Boies on behalf of Microsoft.

Cheffo says that Microsoft CEO Satya Nadella had allegedly emailed Bastian, “who has never replied.” Meanwhile, CrowdStrike also has claimed CEO George Kurtz had reached out to Bastian “but received no response.”

“Even though Microsoft’s software had not caused the CrowdStrike incident, Microsoft immediately jumped in and offered to assist Delta at no charge following the July 19th outage,” the letter from Cheffo reads.

“Each day that followed from July 19th through July 23rd, Microsoft employees repeated their offers to help Delta. Each time, Delta turned down Microsoft’s offers to help, even though Microsoft would not have charged Delta for this assistance.”

Delta’s problems continued through July 25 following the July 19 outage, forcing the carrier to cancel more than 5,000 flights. The incident cost Delta Air Lines around $500 million, CEO Ed Bastian said last week on CNBC.

Like CrowdStrike, Microsoft is now demanding Delta keep all relevant files relating to its tech infrastructure and dealings with other companies, such as Amazon and IBM, with Microsoft claiming Delta’s internal IT is dated as Cheffo says it is still figuring out why Delta’s competitors, American and United Airlines, were able to bounce back more quickly during the IT failure.

Delta struggled to bring its systems back online longer than other airlines in which Bastian said that some 40,000 servers had to be manually reset. He said getting tracking-related tools back online was particularly crucial because of the important role it played in travel.

“Our preliminary review suggests that Delta, unlike its competitors, apparently has not modernized its IT infrastructure, either for the benefit of its customers or for its pilots and flight attendants,” Cheffo wrote.

On Friday, Bastian told staff that it had been CrowdStrike and Microsoft whose operating systems were affected by the update, that Delta was “planning to pursue legal claims” for losses over the outage and believed there was “no choice” but to seek a legal remedy.

On Sunday, CrowdStrike’s CEO maintained that the company was not responsible for extended Delta flight disruptions following a global IT outage.

The next day on Monday, lawyers for CrowdStrike responded to the threat of legal action by Delta saying in a letter that the airline turned down onsite help from the tech company amid the outage.

Michael Carlinsky, an attorney from Crowdstrike, wrote to Boies that CrowdStrike’s CEO George Kurtz had “personally” offered onsite assistance to Bastian “but received no response.”

The threat of legal action, Carlinsky claimed, “contributed to a misleading narrative that CrowdStrike is responsible for Delta’s IT decisions and response to the outage.”

“Should Delta pursue this path, Delta will have to explain to the public, its shareholders and ultimately a jury why CrowdStrike took responsibility for its actions — swiftly, transparently and constructively — while Delta did not,” he said.