Delta Air Lines Inc. and other carriers worked to get thousands of stranded passengers onto planes after a major electrical disruption in Atlanta crippled service at the world’s busiest hub, extending flight cancellations into a second day.

Hartsfield-Jackson Atlanta International Airport still had 411 inbound and outbound cancellations on Monday after 1,183 the day before, according to FlightAware, a flight-tracking service. Delta, which dominates service at the airport, reported 400 scrubbed flights on Monday. Southwest Airlines Co. said it was operating its full schedule after scraxpping 70 flights Sunday.

While most power was restored around midnight, some operations had yet to return fully as of 9:30 a.m. The SkyTrain connecting the airport with an off-site rental car facility and the underground Plane Train linking concourses were both expected to begin running again around 10 a.m., airport spokesman Reese McCranie said.

The local utility in charge of the electricity supply, Georgia Power Co., said a fire caused “extensive damage” in an underground facility, though no passengers or workers were in any danger.

Atlanta Mayor Kasim Reed said the intensity of the fire damaged the switch that would have allowed the built-in backup power systems to take over. Complicating matters further, the heat and fumes in the underground tunnels prevented Georgia Power crews from starting work on repairs for some time and lengthened the disruption, he said at a Sunday night news conference.

There is “no evidence to suggest the fire was caused deliberately,” Reed said.

‘Essential Activities’

Georgia Power said in a statement Monday that all “essential activities” at the airport, including all concourses and flight operations, have power.

The electricity went out after 1 p.m. Sunday, prompting the Federal Aviation Administration to issue a stop on all flights heading to Hartsfield-Jackson. Aircraft were held at their departure airports and planes in the air were diverted to other cities, the airport said.

At one point, Delta had almost 100 planes that were unable to park at gates because of the power power failure, an airline spokesman said. Airlines were still retrieving bags misplaced in Sunday’s chaos.