Airlines including Delta Air Lines were still struggling to resume flights, two days after a faulty software update caused technological chaos worldwide and forced several airlines to ground flights.

The total number of cancellations within, to or from the U.S. on Sunday was 1,461, according to the latest data from FlightAware. Delta and United Airlines topped the list of cancellations.

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

Bastian noted that one of their crew registration tools had been compromised and was unable to effectively handle the unprecedented volume of changes caused by the system outage.

“The technology issue occurred during the busiest travel weekend of the summer, with our booked loads exceeding 90%, limiting our relocation options,” Bastian wrote. “I want to apologize to everyone affected by these events.”

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

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

“No one should be stranded at an airport overnight or have to wait on hold for hours to speak to a customer service representative,” Buttigieg said. “I will ensure our department supports Delta’s customers by enforcing all applicable passenger protections.”

The outages affected major companies including Visa, Amazon, Delta and United.