A class action lawsuit is pending against Delta Air Lines, alleging that the airline refused to issue refunds following a global technology outage last month.

Delta was by far the most affected airline by the outage and had to cancel thousands of flights because important systems were paralyzed by the incident.

The lawsuit was filed Tuesday in the U.S. District Court for the Northern District of Georgia (Atlanta Division) on behalf of Delta customers affected by the outage. In it, the customers claim that Delta denied or ignored their requests for immediate refunds for their canceled or delayed flights.

The complaint also alleges that Delta failed to provide vouchers for meals, hotels and ground transportation to all affected passengers and continues to deny or ignore requests for reimbursement for these unexpected expenses.

“These unfair, unlawful and unconscionable practices resulted in Delta unjustly enriching itself at the expense of its customers,” the lawsuit states.

The complaint states that the plaintiffs are seeking refunds for all Delta customers whose flights were canceled or significantly impacted due to the outage.

Delta, based in Atlanta, did not immediately respond to a request for comment.

The U.S. Department of Transportation is currently investigating why Delta has not recovered as quickly as other airlines. Transportation Secretary Pete Buttigieg said last month the department would also investigate Delta’s customer service, including “unacceptable” lines for assistance and reports that unaccompanied minors were stranded at airports.

On Tuesday, Microsoft announced it was teaming up with cybersecurity software firm CrowdStrike to fight Delta, which blames the companies for several thousand canceled flights following the outage in July.

A Microsoft lawyer said Delta’s core IT system is likely maintained by other technology companies, not Microsoft Windows.

Delta CEO Ed Bastian had previously said the global technology outage, which began with a faulty CrowdStrike upgrade to machines running Microsoft Windows, cost the airline $500 million. Bastian threatened legal action.

On Tuesday, Delta said it has a long history of investing in reliable service, including “billions of dollars in IT investments” since 2016 and billions more in annual IT costs. The company declined further comment.

CrowdStrike has also denied Delta’s claims. Both CrowdStrike and Microsoft said Delta declined their offers to help the airline recover from last month’s outage. Microsoft’s lawyer said CEO Satya Nadella emailed Bastian during the outage, but the Delta CEO never responded.