CrowdStrike said it has now sued Delta Air Lines in US District Court in Georgia after a faulty software update prompted a global outage in July.
The July 19 incident led to worldwide flight cancellations and hit industries including banks, healthcare, media companies and hotel chains.
CrowdStrike said it sued to make clear that CrowdStrike did not cause the harm that Delta claims, and that Delta repeatedly refused assistance from both CrowdStrike and Microsoft.
Delta did not immediately comment on CrowdStrike's suit.
CrowdStrike is seeking a declaratory judgment plus legal fees.
Delta filed its own lawsuit in Fulton County Superior Court late last week, calling the faulty software update from CrowdStrike "catastrophic".
It said the company "forced untested and faulty updates to its customers, causing more than 8.5 million Microsoft Windows-based computers around the world to crash."
Delta said the faulty update caused 7000 flight cancellations, disrupted travel plans of 1.3 million customers and cost the carrier more than US$500 million ($759 million).
CrowdStrike's lawsuit, also filed at the same time but not previously known, said Delta's own response and technology caused delays in the carrier's ability to resume normal operations.
CrowdStrike's lawsuit reiterated its contention that it has minimal liability, something Delta rejected.
Delta said CrowdStrike is liable for over US$500 million in out-of-pocket losses as well as for unspecified lost profits, expenditures - including legal fees - reputational harm and future revenue loss.
The incident prompted the US Transportation Department to open an investigation.
"If CrowdStrike had tested the faulty update on even one computer before deployment, the computer would have crashed," Delta's lawsuit says.
Delta said it has invested billions of dollars in information technology licensing and infrastructure.
Last month, a senior CrowdStrike executive apologized before Congress for the faulty software update.
