STORY: Microsoft blamed Delta Air Lines on Tuesday (August 7) for its struggle to recover from a global cyber outage last month.
The days-long issue led the airline to cancel more than 6,000 flights.
A software update in July by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers, including many airlines.
But disruptions eased the next day at other major U.S. carriers - while problems persisted at Delta.
Microsoft said its preliminary review suggested that Delta had not modernized its IT infrastructure, unlike its competitors.
Delta hit back, saying it has invested billions of dollars in IT capital expenditures since 2016, in addition to the billions it spends annually on operating costs.
Last week Delta's CEO Ed Bastian branded Microsoft's platform the "most fragile" of the two the airline relies on.
Microsoft has refuted this statement.
The flight disruptions stranded hundreds of thousands of travelers and are estimated to cost the Atlanta-based airline $500 million.
Delta is also facing an investigation from the U.S. Transportation Department for the disruptions.
It has hired a prominent litigator to seek damages from both CrowdStrike and Microsoft.
Both companies rejected Delta's claim that CrowdStrike should be blamed for the flight disruptions.
And both companies also said they had immediately offered to assist Delta during the incident.
Microsoft said the U.S. carrier turned them down, while CrowdStrike said they received no reply.
