Microsoft criticized Delta Air Lines today for overstating the technology company’s role in a costly disruption that led the airline to cancel thousands of flights last month.

In a letter between lawyers representing the two companies, which was reviewed by The New York Times, Microsoft said Delta had sought to deflect from its own shortcomings by blaming the technology giant for its problems.

Delta’s comments, both publicly and in legal correspondence, are “incomplete, false, misleading and damaging,” Microsoft said in the letter, which was addressed to David Boies, a prominent lawyer whose firm is representing Delta.

“The truth is very different from the false picture you and Delta have sought to paint,” wrote Mark Cheffo, a partner at the Dechert law firm who is representing Microsoft.

In mid-July, a cybersecurity company, CrowdStrike, issued a flawed software update to customers who run Microsoft Windows, effectively shutting down various computer systems of lots of businesses, including several airlines. Most carriers bounced back relatively quickly, but Delta struggled for days, ultimately canceling about 5,000 flights over four days, or more than one-third of its schedule.

Delta’s chief executive, Ed Bastian, has said that the episode cost the airline about $500 million. Last week, he told employees that he had hired Boies’ firm, Boies Schiller Flexner, to pursue legal claims against Microsoft and CrowdStrike, which also rebutted Delta’s claims this week.

In its letter today, Cheffo said that Microsoft “empathizes” with Delta and its customers and said that while the technology company was not at fault, it had offered to help the airline at no charge after the outage. Microsoft repeated that offer over five days, from July 19 to July 23, but was turned down each time, it said.

The technology company’s chief executive, Satya Nadella, also reached out to Bastian on July 24, but never received a response, according to the letter. (In its letter, CrowdStrike said Delta had rejected or ignored its offers for help, too.)

Microsoft said that Delta probably turned it down because airline was struggling the most with a computer system it uses to track and schedule the airline’s crew that was being serviced by other companies, including IBM. It also accused Delta of using outdated information technology.

“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,” Microsoft said in the letter.

The airline rebutted that suggestion in a statement today.

“Delta has a long track record of investing in safe, reliable and elevated service for our customers and employees,” the company said. “Since 2016, Delta has invested billions of dollars in IT capital expenditures, in addition to the billions spent annually in IT operating costs.”

Microsoft demanded that Delta preserve various documents related to the disruption, saying that Microsoft would “vigorously defend itself” if the airline filed a lawsuit.

This article originally appeared in The New York Times.

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