In a letter sent to Delta, Microsoft echoes claims by CrowdStrike that Delta ignored offers to help with recovery from the July 19 outage.

Microsoft has accused Delta of ignoring offers to help with recovery from the massive Windows outage last month in part because its IT systems were allegedly outdated.

The criticism of Delta echoed sentiments in a letter sent to the airline by CrowdStrike—whose faulty update caused the July 19 outage—which likewise indicated that Delta turned down offers to assist. Delta canceled thousands of flights and says it lost $500 million over five days following the outage.

[Related: CrowdStrike: More Testing, Staged Rollouts Now In Place For Updates]

The letter sent by Microsoft to Delta, which was provided to media outlets including CRN Tuesday, includes the allegation that “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.”

Microsoft said it has been investigating the incident in part to “understand why other airlines were able to fully restore business operations so much faster than Delta, including American Airlines and United Airlines,” Microsoft said in the letter. “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.”

The letter indicates that, amid the outage, Delta executives “repeatedly” ignored outreach from Microsoft. That included Delta CEO Ed Bastian, who did not reply to a personal email from Microsoft CEO Satya Nadella offering assistance, according to the Microsoft letter.

Microsoft declined to comment further.

In a statement Tuesday, Delta said it 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,” the company said in the statement provided to CRN.

A CNBC report last week indicated that Delta may be preparing to file lawsuits against both CrowdStrike and Microsoft over the outage. Later in the week, during an interview on CNBC, Bastian answered that “we have no choice” after being asked about the potential for a lawsuit.

“We’re looking to make certain that we get compensated,” he said.

In the CrowdStrike letter to Delta, the security vendor said that it attempted to assist the airline with recovering from the outage, but the offer was not accepted.

“Within hours of the incident, CrowdStrike reached out to Delta to offer assistance and ensure Delta was aware of an available remediation,” CrowdStrike’s attorney wrote in the letter. “Additionally, CrowdStrike’s CEO personally reached out to Delta’s CEO to offer onsite assistance, but received no response. CrowdStrike followed up with Delta on the offer for onsite support and was told that the onsite resources were not needed.”

Delta was by far the hardest-hit airline in the outage, which continued to have a major impact on its flights for five days. Two other airlines that initially saw significant disruptions from the outage, United and American Airlines, recovered more quickly than Delta.

Amid the outage, the airline said in a post online that “upward of half” of its IT systems run Windows. The airline blamed the issues, at least in part, on “one of our crew tracking-related tools [that] was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown.”