Alexis Canete rests on his luggage on Dec. 18 as he waits in the Delta ticket line to get back home to Cuba at Hartsfield-Jackson International Airport in Atlanta. (John Spink/AP)

The FBI is part of the probe into what caused a fire that knocked out power to the world's busiest airport in Atlanta, but an agency spokesman said Tuesday that there was no sign of anything connected to terrorism.

"There's no indication at this point of anything nefarious," FBI spokesman Kevin Rowson said.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has also been involved in the investigation, Georgia Power spokesman Craig Bell said.

"We're bringing everything we have to bear to the situation to make sure this doesn't happen again," Bell said Tuesday.

No conclusions have been drawn as to the cause of the fire, which took out the airport's power supply and its backup electricity for about 11 hours Sunday. The blackout stranded thousands of passengers on grounded jets and in darkened concourses and led to the cancellation of more than 1,500 flights just ahead of the frenzied holiday travel period.

Hartsfield-Jackson Atlanta International Airport, the main hub for Delta Air Lines, is a crucial cog in the nation's air travel system. Delays there typically ripple across the nation because so many U.S. and international flights are routed through the Atlanta hub.

Because of the magnitude of Sunday's outage, "we want to be able to rule out any possible scenario that wasn't equipment malfunction," Bell said.

"We really don't expect any answers like that to come forth for a few days," he said.

The power company is working with the airport to explore how to prevent the situation from happening again.

Among ideas being discussed are encasing in concrete the area that holds key electric equipment or moving parts of the system to other areas. The blaze took out the main power and the backup system because the fire burned through parts of both in the same underground utility tunnel, authorities have said.

Delta and other carriers said they expected to be running normally Tuesday. But passengers trying to catch Tuesday morning flights faced wait times of up to an hour just to get through the main security checkpoint in the domestic terminal, the airport's website showed.

On Tuesday — two days after the outage — passengers were still sleeping in the atrium area that's often used for events aimed at showcasing the world's busiest airport. Video from news outlets showed passengers sprawled out on benches and chairs and luggage piled up in a nearby area of the domestic terminal.

Hartsfield-Jackson serves an average of 275,000 passengers a day. Nearly 2,500 planes arrive and depart each day.