ATLANTA -- Georgia Power said Monday that a "piece of switchgear" in an underground substation appears to be "involve[d]" in the start of a fire that shut down Atlanta Hartsfield-Jackson Airport Sunday.

The blackout was rooted in a Georgia Power substation located underground between concourses E and F— igniting wires and switches that power the airport and control its backup systems.

"The fumes were so poisonous that we could not access the facilities that are under the airport to determine what was wrong," said Mayor Kasim Reed Monday. "We then had to use a series of blowers and fans to clear that corridor so Georgia power employees could access it."

Workers were able to access it some three and a half hours after the blackout started. The catastrophic damage occurred to a switch that should have activated a different substation on the airport property, but couldn’t.

"With the failure of the switch gear, it actually took both sources out. The intense heat took the wire that’s mounted on the walls out of service for us," said Georgia Power CEO Paul Bowers in an interview with 11Alive News Monday.

Airport General Manager Roosevelt Council Jr. said he hadn't realized such a blackout was possible at the airport. "The redundant (power) system has been operating here for quite a while. We’ve never had a problem with it," Council said in an interview. "It has been performing great for us."

Watch video of the damage:

Georgia Power says it's still unclear what ignited the fire.