Electricity at Atlanta's Hartsfield-Jackson International was out for nearly 11 hours on Sunday
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A massive power outage at Atlanta’s Hartsfield-Jackson International caused more than 1,000 flights to be canceled on Sunday and left many passengers stranded at the terminal or on the tarmac for hours.

But what caused the Atlanta airport power outage? Officials say the power outage was caused by a fire at an electrical facility that services the world’s busiest airport.

Atlanta Mayor Kasim Reed said the airport lost power because a fire broke out in an underground Georgia Power electrical facility. Reed said that fire damaged two substations connected to the airport, which also affected the airport’s backup "redundant system," CNN reports.

Georgia Power learned of the fire at 12:38 p.m. on Sunday, according to CNN, but their employees couldn’t attempt to address the fire immediately because the heat and flame intensity was too great in the underground tunnel.

In a statement, Georgia Power tentatively attributed the fire to an equipment failure in a switchgear piece, and said it will continue to probe the cause.

The hours-long Atlanta airport power outage caused bedlam during the busy holiday travel season on Sunday, with the Federal Aviation Administration suspending incoming flights.

TSA officers began screening passengers to resume boarding again when the checkpoints opened at 3:30 a.m. Monday morning. However, hundreds of flights are still facing delays and cancellations as of Monday, according to FlightAware.