FAA says 'unintentionally deleted files' led to NOTAM system outage

The Federal Aviation Administration said Thursday that last week's computer system outage, which caused thousands of flight delays across the United States and a temporary grounding of the nation's airspace, appears to have been the result of "unintentionally deleted files."

"A preliminary FAA review of last week’s outage of the Notice to Air Missions (NOTAM) system determined that contract personnel unintentionally deleted files while working to correct synchronization between the live primary database and a backup database," the FAA said in a statement. "The agency has so far found no evidence of a cyber-attack or malicious intent. The FAA continues to investigate the circumstances surrounding the outage."

"The FAA made the necessary repairs to the system and has taken steps to make the NOTAM system more resilient," the FAA added. "The agency is acting quickly to adopt any other lessons learned in our efforts to ensure the continuing robustness of the nation’s air traffic control system."

The FAA's NOTAM system is critical to flight operations -- it keeps pilots informed of essential information that's needed before takeoff, such as runway conditions at destination airports, weather en route and even real-time safety alerts during flight. Notably, the system is overdue for replacement.

When the FAA first reported an issue with the NOTAM system late in the night on Jan. 10, it led to a "cascading" series of IT failures culminating in the nationwide disruption the next morning, a senior official briefed on the matter official told ABC News. By mid-day, there were more than 7,300 flight delays and 1,100 cancellations, according to tracking website Flight Aware.

The official compared the NOTAM system outage to the crisis that crippled Southwest Airlines over the winter holidays: antiquated software overdue for replacement inside a critical IT network. If one thing goes down, the system can become paralyzed.

The issue apparently occurred during routine scheduled systems maintenance, a senior official briefed on the internal review told ABC News. An engineer "replaced one file with another," the official said, not realizing the mistake was being made. As the NOTAM system began showing problems and ultimately failed, FAA staff feverishly tried to figure out what had gone wrong. Engineers and IT teams are now working to keep the system from crashing again, while also trying to figure out if there are any similar systems that could fail without redundancies.

Congressional hearings on the matter are expected in the coming weeks, as is a possible speed-up of system replacement.

Several airlines, including American, Delta and United, are waiving fees to rebook flights due to last week's disruption.

ABC News' Amanda Maile, Josh Margolin and Sam Sweeney contributed to this report.