New York CNN  —  Microsoft on Tuesday blamed Delta Air Lines for its weeklong bout of cancelations last month, claiming its aging technology caused the airline to recover far slower from the global tech outage than its rivals. Microsoft also claimed Delta turned down help after the CrowdStrike bug led to “blue screens of death” on Windows devices. Microsoft, CrowdStrike and Delta have been in a war of words since the airline hired a high-profile attorney to seek compensation from Microsoft and CrowdStrike. Delta CEO Ed Bastian lashed out at CrowdStrike in a CNBC interview last week and said the computer problems cost Delta $500 million. CrowdStrike’s flawed software update caused widespread computer outages on July 19 at Delta and hundreds of other companies around the globe. CrowdStrike then fired back on Sunday, saying the airline had refused offers of help to get through the outage faster. On Tuesday, Microsoft joined in. In a letter by attorney Mark Cheffo, the tech company said that even though Microsoft’s software did not cause the outage, it “immediately jumped in and offered to assist Delta at no charge.” Microsoft employees asked Delta if they needed assistance every day from July 19 to 23, the letter said. “On the morning of July 22, a Microsoft employee, aware that Delta was having more difficulty recovering than any other airline, messaged a Delta employee to say, ‘just checking in and no pressure to reply, but if you can think of anything your Microsoft team can be helping with today, just say the word.’” The letter continued. “The Delta employee replied, saying ‘all good. Cool will let you know and thank you.’” But meanwhile, the computer problems at Delta knocked its crucial crew tracking system offline for the better part of a week, making it impossible for the company to find the pilots and flight attendants it needed to fly its aircraft. While other airlines were quick to resume normal operations after the CrowdStrike outage, Delta was forced to cancel about 30% of its schedule over those five days, leaving an estimated half-million passengers stranded. It took many days after that to re-book affected passengers on other flights and return their checked bags. On July 24, Microsoft CEO Satya Nadella emailed Bastian as well, according to the letter. “In fact, it is rapidly becoming apparent that Delta likely refused Microsoft’s help because the IT system it was most having trouble restoring—its crew-tracking and scheduling system—was being serviced by other technology providers, such as IBM, because it runs on those providers’ systems, and not Microsoft Windows or Azure,” the letter said. Microsoft said a preliminary review suggested Delta hasn’t “modernized its IT infrastructure” though its competitors have. “Delta has a long track record of investing in safe, reliable and elevated service for our customers and employees. Since 2016, Delta has invested billions of dollars in IT capital expenditures, in addition to the billions spent annually in IT operating costs,” a Delta spokesperson said. Who is responsible for the travel woes? Last week, Bastian claimed CrowdStrike was nowhere to be found during the meltdown. “They haven’t offered us anything. Free consulting advice to help us,” said the Delta CEO in an interview on CNBC. “If you’re going to have priority access to the Delta ecosystem in terms of technology, you’ve got to test this stuff,” Bastian said. “You can’t come into a mission critical 24/7 operation and tell us we have a bug. It doesn’t work.” A person familiar with its actions confirmed to CNN that Delta had hired the law firm of high-profile attorney David Boies to pursue compensation from Microsoft and CrowdStrike. “We have no choice,” Bastian told CNBC. “We have to protect our shareholders, we have to protect our customers (and) our employees for the damage, not just the cost but the reputational damage.” On Monday, CrowdStrike’s legal counsel sent a letter to Delta’s legal counsel, saying it was “highly disappointed by Delta’s suggestion that CrowdStrike acted inappropriately and strongly rejects any allegation that it was grossly negligent or committed willful misconduct.” Like Microsoft, CrowdStrike CEO George Kurtz offered onsite assistance personally to Delta CEO Ed Bastian but received no response, the letter said, adding that Delta later told CrowdStrike no help was needed. Delta’s public litigation threat “contributed to a misleading narrative that CrowdStrike is responsible for Delta’s IT decisions and response to the outage,” attorney Michael Carlinsky wrote in the letter, “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.” The letter also said Delta would have to explain “resiliency capabilities of Delta’s IT infrastructure.” Delta has not yet filed a lawsuit against Microsoft or CrowdStrike. In a statement, a CrowdStrike spokesperson said the letter speaks for itself. “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 long-time partner is not constructive to any party. We hope that Delta will agree to work cooperatively to find a resolution,” a CrowdStrike spokesperson said.