Facepalm: Making a mistake at work is something most people have experienced, though the consequences are unlikely to match those of FAA contract personnel who, by accidentally deleting essential files, caused a nationwide grounding of flights.

On January 11, the Federal Aviation Administration announced that its Notice to Air Mission Systems (NOTAM) system was offline, leading it to order airlines to pause all domestic departures—the first nationwide grounding since September 11, 2001.

Now, the FAA has revealed the cause of the problem. The agency announced that following a preliminary review, it was determined that contract personnel unintentionally deleted files while working to correct synchronization between the live primary database and a backup database. The FAA is continuing to investigate the incident, but it has so far found no evidence of cyberattack or malicious intent.

The NOTAM system informs pilots of essential information required before takeoff, including runway conditions at destination airports, weather, and real-time safety alerts. A senior FAA official said the issue with the system led to a "cascading" series of IT failures and nationwide disruption.

The problems began during routine scheduled systems maintenance when an engineer "replaced one file with another," a mistake that resulted in 11,000 flights being grounded. The FAA never went into detail regarding the nature of the file and if any data had been lost, though it has taken steps to make the system more resilient. "The agency is acting quickly to adopt any other lessons learned in our efforts to ensure the continuing robustness of the nation's air traffic control system," the FAA said in a statement.

The outage brought the ire of more than 120 Congressional representatives who wrote to the FAA seeking answers about the system crash. The letter highlights the series of flight delays and mass cancellations over the last year that have caused considerable harm to passengers. It claims that some of these have been due to the FAA's apparent issues with managing the agency's air traffic control legacy systems.

Congressional hearings on the matter are expected to arrive soon. ABC News notes that the NOTAM system is overdue for replacement, and these hearings could speed up the process.