Delta CEO Ed Bastian told staff on Friday that it had told CrowdStrike and Microsoft, whose operating systems were impacted by the update, that it was “planning to pursue legal claims” for losses over the outage after he told CNBC that the company had “no choice” but to do so.

Michael Carlinsky, an attorney from Crowdstrike, wrote in a letter to Delta’s attorney David Boies that CrowdStrike’s CEO George Kurtz had “personally” offered onsite assistance to Bastian “but received no response.”

He said the threat of legal action “contributed to a misleading narrative that CrowdStrike is responsible for Delta’s IT decisions and response to the outage.”

“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,” Carlinsky said.

Delta’s problems continued through July 25, forcing the carrier to cancel more than 5,000 flights. Bastian said he plans legal action against CrowdStrike to recoup $500 million in losses.

Delta struggled to bring its systems back online longer than other airlines where Bastain said that some 40,000 servers had to be manually reset. He said getting tracking-related tools back online was particularly crucial because of the important role it played in travel.

The U.S. Department of Transportation said it would investigate Delta’s prolonged flight troubles to ensure it was “following the law and taking care of its passengers.”

CrowdStrike is also facing litigation from shareholders over falling stock prices following the outage as they allege the company’s statements about its Falcon software program were false and misleading.