The Federal Aviation Administration (FAA) said on Thursday a preliminary review found that contract personnel unintentionally deleted files prompting a nationwide groundstop on Jan. 11 that disrupted more than 11,000 flights.

The FAA said the issue occurred while personnel were working “to correct synchronization between the live primary database and a backup database” and the FAA added that it “has so far found no evidence of a cyber-attack or malicious intent.”

The FAA said it has made necessary repairs to the system “and has taken steps to make the pilot message system “more resilient.”