The Federal Aviation Administration believes it knows what caused a computer outage that forced flights to remain grounded last week.

The agency says a preliminary review determined that “contract personnel unintentionally deleted files while working to correct synchronization between the live primary database and a backup database.”

The FAA adds that it has found no evidence of a cyberattack or malicious intent.

Most airports in the U.S. held planes at the gate for about two hours on the morning of Jan. 11 while the situation was resolved. Planes were still allowed to land.