Total cancellations within, into or out of the U.S. on Sunday clocked in at 1,461, according to the latest data from FlightAware. Delta and United Airlines topped the cancellations.

Delta Chief Executive Ed Bastian said in a message to customers Sunday that flight cancellations continued as the airline tried to recover its systems and restore operations. He noted that the pause in Delta’s operations resulted in more than 3,500 Delta and Delta Connection scrubbed flights. Delta has been offering waivers to affected customers.

Bastian noted that that one of their crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system outage.

“The technology issue occurred on the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our re-accommodation capabilities,” Bastian wrote. “I want to apologize to every one of you who have been impacted by these events.”

Transportation Secretary Pete Buttigieg spoke to Bastian on Sunday about the airline’s high number of cancellations since Friday. The Transportation Department said its top officials have reminded Delta of the airline’s obligation to provide refunds to passengers whose flights were canceled and who don’t want to be rebooked on a later flight.

“I have made clear to Delta that we expect the airline to provide prompt refunds to consumers who choose not to be rebooked, and free rebooking and timely reimbursements for food and overnight hotel stays to consumers affected by the delays and cancellations, as well as adequate customer service assistance to all of their passengers,” Buttigieg said.

“No one should be stranded at an airport overnight or stuck on hold for hours waiting to talk to a customer service agent,” Buttigieg said. “I will ensure that our department supports Delta passengers by enforcing all applicable passenger protections.”