A failure in a Federal Aviation Administration computer system that processes flight plans delayed hundreds of flights nationwide Tuesday, though the FAA expected to have the problem fixed by the evening.

"Our estimate is a return to normal ops in an hour in Atlanta," Hank Krakowski, chief operations officer for the FAA, told reporters Tuesday afternoon. "The airlines have been working quite well with us. It looks like we're slowly starting to dig out of this."

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Shortly after 1 p.m., the National Airspace Data Interchange Network in Atlanta failed, and all flight plans were routed to the NADIN facility in Salt Lake City, bogging down the logging of flight plans and temporarily grounding hundreds of flights in airports all along the East Coast.

"The system that processes flight plans is backed up," said FAA senior spokeswoman Laura Brown. "Planes must file a flight plan to take off. Without that, they sit on the ground."

Brown said bad weather at the Atlanta airport was complicating the situation. Pilots affected by the outage in Atlanta were forced to refile their flight plans to Salt Lake City, causing further delays.

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FAA officials said they were not sure what had caused the initial failure but were investigating. "Hacking does not appear to be an area (of concern)," said Krakowski. "Right now it looks like an internal software processing failure in the area. We actually think we know what it is; we're in the process of determining what it was right now."

Most delays have been cleared up, but Chicago's Midway airport was experiencing delays of about 30 minutes for all departing flights late Tuesday afternoon, O'Hare International Airport was delaying flights by 60 to 80 minutes, Baltimore/Washington International Airport was experiencing residual delays and flights from Atlanta were delayed 90 minutes, according to FAA spokesmen.

"Most of other airports have been cleared up," Brown said.

A number of airports along the East Coast remained unaffected, as flights were taking off and landing without a problem at Miami International airport and elsewhere.

The Department of Homeland Security confirmed that the delays were not terror-related.

"There is no indication of a terrorism nexus at this time," said DHS spokesman Russ Knocke. "We are monitoring the FAA computer problem, and we remain in close contact with DOT as they work to resolve the problem."

FAA spokeswoman Kathleen Bergen said officials were still able to speak to pilots on planes on the ground and in the air.

She says she doesn't know how many flights are being affected.