Last week’s massive flight disruption that grounded thousands of planes was caused after “contract personnel unintentionally deleted files,” the Federal Aviation Administration said Thursday.On Jan. 11, the FAA halted all departures nationwide after an outage to the Notice to Air Missions, or NOTAM, system.The FAA said in a statement that the files were deleted by accident as the contractors were "working to correct synchronization between the live primary database and a backup database."On Jan. 11, the FAA sent out a tweet at 7:20 a.m. ordering the airlines to pause all domestic departures until 9 a.m.The agency said Thursday that repairs have been made to make the system more resilient and that it will adopt other steps."