The Federal Aviation Administration said Thursday that contractors working on a system that notifies pilots of potential hazards “unintentionally deleted files,” leading to a nationwide grounding of planes and thousands of delays last week.

The agency said it was continuing to investigate and had updated Congress on the latest findings as lawmakers demand answers about the cause of the outage.

The system distributes bulletins called Notices to Air Missions and is a patchwork of new technology and components that are three decades old. In its statement Thursday, the FAA said the contract workers were trying to correct synchronization between a live database and a backup system when the problem began.

The system started to fail on the afternoon of Jan. 10, and efforts to restore it faltered into the evening. In the early hours of Jan. 11, the agency decided to reset the system and order a nationwide halt to air travel – the first of its kind since 9/11. Air traffic soon began moving again, but delays continued throughout the day.

The FAA said its preliminary review found no evidence of a cyberattack or malicious intent.

The outage highlighted the aging computer systems that aviation safety relies upon. It remains unclear how the blunder could bring down the entire notification system, but the FAA said it has fixed the system and taken steps to ensure it is 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.

The outage came after Southwest Airlines had thousands of flight cancellations in December, a problem it blamed on outdated technology.

The two incidents have heightened scrutiny on the aviation system in Congress as lawmakers prepare to craft a multiyear funding package for the FAA.

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