FAA says unintentionally deleted files are to blame for nationwide ground stop

The agency determined that a contractor working for the FAA was responsible for the deletion of files related to a key pilot safety system.

(CNN) -- A contractor working for the Federal Aviation Administration unintentionally deleted files related to a key pilot safety system, leading to a nationwide ground stop and thousands of delayed and canceled flights last week, the FAA said Thursday.

The FAA determined the issue with the Notice to Air Missions (NOTAM) system occurred when the contractor was "working to correct synchronization between the live primary database and a backup database."

The NOTAM system sends alerts to pilots to let them know of conditions that could affect the safety of their flights. It is separate from the air traffic control system that keeps planes a safe distance from each other, but it's another critical tool for air safety. NOTAM messages could include information about lights being out on a certain runway, or a tower near an airport not having the required safety lights working, or an air show taking place in the air space nearby.

CNN previously reported that the FAA said "personnel who failed to follow procedures" caused the computer system failure that triggered the delay. Now, the FAA is providing an additional detail about last week's problem with its NOTAM system that grounded air traffic across the country, saying unintentionally deleted files were to blame.

The NOTAM database failure triggered the FAA to implement the first nationwide stop of air traffic in more than 20 years.

The FAA has also found no evidence of a cyber-attack or malicious intent. The agency says it has taken steps to make the system more resilient.

CNN's Pete Muntean, Gregory Wallace and Chris Isidore contributed to this report

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