The Federal Aviation Administration (FAA) is investigating contractor L3Harris Technologies after a telecommunications failure disrupted air traffic control systems across the country, forcing a nationwide ground stop Friday and leaving thousands of travelers facing cancellations and delays.

Transportation Secretary Sean Duffy identified L3Harris as the contractor performing maintenance at a remote FAA facility when the outage occurred. Officials are investigating whether an error during the maintenance work or faulty equipment caused the failure, which disrupted communications between air traffic controllers and pilots and affected radar systems.

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More than 4,100 flights were canceled Saturday, according to FlightAware data cited by The Associated Press. That followed 9,132 cancellations Friday, according to an earlier FlightAware update reported by ABC News. The figures continued changing Saturday as airlines worked to restore their schedules.

The FAA lifted the nationwide ground stop at approximately 8:15 p.m. Eastern time Friday. Although departures have resumed, airlines are still working to reposition aircraft and crews and accommodate passengers whose travel plans were disrupted.

L3Harris contractor work under investigation

During a news conference Saturday, Duffy outlined what officials believe led to the failure. A contractor was performing maintenance at a remote FAA facility, replacing older copper telecommunications lines with fiber-optic connections.

Duffy identified the contractor as L3Harris, which he described as the FAA’s longtime telecommunications provider. He said the work involved a router that began overwhelming other parts of the air traffic control network.

“Either through contractor error or a faulty piece of equipment, that router that was being replaced as we switched from copper to fiber, it started flooding other routers in our air traffic control system,” Duffy said.

He said the system experienced more than 1,000 connection attempts every four to five seconds, disrupting communications with aircraft and causing problems with radar feeds.

Duffy said officials are investigating whether the problem resulted from contractor error or a faulty router.

“We haven’t determined what the fault was,” he said.

L3Harris had not responded to a request for comment at the time of the latest reporting. Officials have not determined that the company was responsible for the outage.

The FAA initially confirmed Friday that it had issued a nationwide ground stop because of communication problems affecting multiple Air Route Traffic Control Centers.

In its official statement, the agency said it had issued the ground stop following communication issues affecting multiple centers and that FAA technicians were investigating.

Federal officials have also addressed concerns about a possible cyberattack or interference from the FAA’s newer technology. Duffy said the agency does not believe the incident was caused by a cyberattack after technicians isolated the work being performed at the remote facility.

He also said the FAA’s new Strategic Management of Airspace (SMART) system was not connected to the equipment involved in the failure.

Thousands of cancellations continue as airlines recover

The disruption began Friday at approximately 3:36 p.m. Eastern time after air traffic control centers experienced instability involving radar and communications systems.

Duffy said more than 3,500 aircraft were in the air when the FAA issued the ground stop. Air traffic controllers used established safety procedures to guide aircraft to the ground.

The FAA lifted the restriction at approximately 8:15 p.m. Eastern time, allowing departures to resume. However, the disruption left aircraft and flight crews out of position, creating additional cancellations and delays Saturday.

FlightAware reported more than 4,100 cancellations Saturday, while Cirium reported more than 3,200, according to The Associated Press. The difference reflects separate flight-tracking estimates.

Transportation officials acknowledged that restoring normal operations would take time. FAA Administrator Bryan Bedford said Saturday that the nation’s flight schedule should return to normal by the end of Sunday, although additional cancellations and delays were expected.

Duffy said operations were slowly returning to normal Saturday as airlines worked to reposition aircraft and crews.

“You will continue to see some delays and cancellations as carriers reschedule flights and reposition aircraft/crews where they are needed most,” Duffy said in a statement posted Saturday.

Passengers scheduled to travel throughout the weekend should check directly with their airlines for updated departure times and possible cancellations.

Duffy calls for additional funding to modernize air traffic control

The outage has renewed scrutiny of the country’s aging air traffic control infrastructure and the government’s efforts to modernize the system.

Duffy called on Congress to provide additional funding for a broader overhaul of the nation’s air traffic control network, arguing that outdated equipment remains vulnerable to technical failures.

“What we have is an incredibly old air traffic control system,” Duffy said during Friday’s news conference.

Congress has already approved $12.5 billion for modernization efforts, but Duffy said that funding represents only part of the broader project.

The transportation secretary also warned that additional disruptions could occur if the system is not upgraded.

“We didn’t think we would have this kind of outage. We did,” Duffy said Friday. “We were able to review what went wrong, and then reboot the system and go. But you will have more of these.”

For now, the FAA has restored operations following Friday’s ground stop, but the investigation into the equipment failure and L3Harris’ role in the maintenance work continues. Airlines are still working to return to normal schedules, with passengers facing additional disruptions throughout the weekend.