WASHINGTON— Friday's nationwide ground stop started when a contractor updated old Federal Aviation Administration telecommunication lines, Transportation Secretary Sean Duffy said Saturday.

Tens of thousands of passengers across the U.S. are still stranded Saturday after the rare ground stop. Bryan Bedford, the FAA administrator who joined Duffy at Saturday's media briefing, said schedule problems should be resolved by Sunday evening.

Nearly 25,000 flights were delayed, and over 9,000 were cancelled on Friday, according to FlightAware, a flight tracking site. The disruption seeped into Saturday morning with more than 4,000 cancellations recorded, which is 18% of flights departing the U.S., according to data from Cirium, an aviation analytics company.

L3Harris, a long time contractor for the FAA, was doing routine maintenance in at least one remote location changing copper wire to fiber optics, Duffy said. "When I tell you that we're using old equipment, we're using old equipment," he said.

He said the FAA is still investigating the exact error that started flooding other routers in the air traffic control system, and if L3Harris was at all responsible.

"It's pretty straightforward work that we were doing yesterday at, again, a very remote location. So what we're working to determine was whether we had incompetence, meaning was it user error, installator error from L3Harris, or did they install a faulty router?"

In a statement on their website, L3Harris said they have been working with the FAA to modernize communication infrastructure, including converting copper wires to a fiber optic system. The contractor said they have completed more than 70% of those conversions.

"We are supporting the FAA in its investigation to understand the conditions that led to the Nov. 13 ground stop of our nation's air travel," the statement said. "L3Harris takes the nationwide ground stop and its impact on travelers, airlines and air traffic operations seriously."

These updates have been happening in American institutions for decades, Duffy said.

"Virtually every single institution in America has gone from copper to fiber, from analog to digital," Duffy said. "Just maybe the FAA is the last one to be doing this work because the investments haven't been made, but this is not complicated."