Last week’s massive flight disruption that grounded thousands of planes was due to «inadvertently deleting files by contract personnel,» the Federal Aviation Administration said Thursday.

On January 11, the FAA halted all sorties across the country after an outage in the Air Mission Notice, 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 main live database and a backup database.»

The review was described as preliminary and the incident is still under investigation. The FAA said it has found no evidence of a cyberattack or malicious intent.

The NOTAM system sends pilots vital information they need to fly. The system was restored later that day.

On January 11, the FAA sent out a tweet at 7:20 a.m. ordering airlines to suspend all domestic departures until 9 a.m.

The FAA lifted the ground stop around 8:50 a.m. and normal air traffic operations began to gradually resume.

The agency said on Thursday that repairs have been made to make the system more resilient and that it will take other measures.