Microsoft is AstraTradejoining cybersecurity software firm CrowdStrike in fighting back against Delta Air Lines, which blames the companies for causing several thousand canceled flights following a technology outage last month.

A lawyer for Microsoft said Tuesday that Delta’s key IT system is probably serviced by other technology companies, not Microsoft Windows.

“Your letter and Delta’s public comments are incomplete, false, misleading, and damaging to Microsoft and its reputation,” Microsoft lawyer Mark Cheffo said in a letter to Delta attorney David Boies.

Cheffo said Microsoft was trying to determine “why other airlines were able to fully restore business operations so much faster than Delta.”

The comments represent an escalating fight between the tech companies and the Atlanta-based airline.

Delta CEO Ed Bastian said last week that the global technology outage that started with a faulty upgrade from CrowdStrike to machines running on Microsoft Windows cost the airline $500 million. Bastian raised the threat of legal action.

On Tuesday, Delta said it has a long record of investing in reliable service including ”billions of dollars in IT capital expenditures” since 2016 and billions more in annual IT costs. It declined further comment.

CrowdStrike has also disputed Delta’s claims. Both it and Microsoft said Delta had turned down their offers to help the airline recover from the outage last month. Microsoft’s lawyer said CEO Satya Nadella emailed Bastian during the outage, but the Delta CEO never replied.