Microsoft Corp. said Delta Air Lines Inc. turned down repeated offers for assistance following last month’s catastrophic system outage, echoing claims by CrowdStrike Holdings Inc. in an increasingly contentious conflict between the carrier and its technology partners.

Microsoft employees reached out to Delta to give technical support every day from July 19 through July 23, and “each time Delta turned down Microsoft’s offers to help,” according to a letter Tuesday from the technology giant’s attorneys to Delta’s representatives. Microsoft Chief Executive Officer Satya Nadella also personally emailed Delta CEO Ed Bastian and never heard back.

“Even though Microsoft’s software had not caused the CrowdStrike incident, Microsoft immediately jumped in and offered to assist Delta at no charge,” according to the letter, which was signed by Mark Cheffo of Dechert LLP.

The claims, in response to Delta’s hiring of attorney David Boies, heighten the tension after Delta suggested it would try to seek compensation for a breakdown it expects to cost it $500 million this quarter. The airline was slower to recover than competitors after an errant software update from CrowdStrike affected Microsoft systems, creating a cascading effect that led Delta to cancel thousands of flights over several days.

Bastian has criticized both CrowdStrike and Microsoft over the outage, telling CNBC in an interview last week that Microsoft is “probably the most fragile platform within that space” when it comes to large technology providers.

Delta’s extended recovery time stemmed from the outage’s effects on an internal system used to process changes to flights and their crews. That left Delta unable to get its aircraft and personnel properly aligned.

In its letter, Microsoft said those systems don’t use its Windows or Azure products and a review suggests that “Delta, unlike its competitors, apparently has not modernized its IT infrastructure.”

Delta said in an emailed statement Tuesday that it “has a long record of investing in safe, reliable and elevated service for our customers and employees.” The airline said it has invested billions of dollars since 2016 in capital spending and annual operating costs related to IT.

The US Department of Transportation has opened an investigation into Delta’s handling of the breakdown and the airline could face a fine from the agency depending on its conclusions. Southwest Airlines Co. was fined $140 million by DOT in 2023 after a meltdown of its operation in late December 2022 stranded more than 2 million passengers.

Microsoft’s rebuttal comes after CrowdStrike said in a similar letter Sunday that Delta refused offers for assistance. CrowdStrike also said Bastian failed to respond to personal outreach from its CEO, George Kurtz.