The Trump administration says Friday’s four-hour nationwide Federal Aviation Administration (FAA) ground stop that delayed or canceled thousands of flight stemmed from routine maintenance.

At a news briefing on Saturday, the US transportation secretary, Sean Duffy, said that the issues began when a contractor was performing “routine maintenance” at a remote FAA facility to “switch telecommunications lines” from copper wire to fiber optic cable.

“What we’re working to determine is whether we had incompetence,” Duffy said, “or did they install a faulty router.”

He added: “Once we were able to isolate the work that was being done at this remote location, all of our systems came back online,” and the ground stop was lifted.

However, travel chaos continued across the US on Saturday.

At the briefing, the FAA’s head, Bryan Bedford, apologized to passengers and said he believed that the worst of the disruption “is behind us”.

“We will still have delays and cancellations today, but that will set us up for a full recovery tomorrow,” Bedford said. “Based on everything that we’re hearing from our airline partners, they believe that they’ll be able to have full schedule integrity by the close of business on Sunday.”

“I want to reiterate: we see zero evidence of anything nefarious, and zero evidence of anything AI-related,” he continued. “So we want to be clear that we have a equipment issue, and we’re working through what this defective piece of equipment was.”

Even as Duffy and Bedford spoke on Saturday, airlines worked to recover from Friday’s four-hour nationwide Federal Aviation Administration (FAA) ground stop that delayed or canceled thousands of flights.

By of 11am ET on Saturday morning, more than 4,000 flights within, into or out of the US had already been canceled, according to FlightAware, adding to the more than 8,000 cancellations recorded on Friday.

The disruption, which marked the longest nationwide ground stop in 25 years, left passengers stranded at airports across the country and facing uncertainty about when they could reach their destinations.

Duffy on Friday had blamed the ground stop on an ageing air control system, saying that the communications equipment became “overloaded with data” and was starting to cause inconsistency in the data”.

“Because of that, we shut the system down,” Duffy said.

Duffy on Saturday again mentioned the US having a “decades-old air traffic control system”. “It’s not just old equipment – we have an incredibly old telecommunications system,” he said.

“The work happening was due to the age of the copper wire that we were replacing,” Duffy said on Saturday. “What we’re working determine was whether … it [was] user error or installer error.”

Duffy echoed Bedford in saying “we don’t believe” the disruption resulted from a cyber-attack or some hostile foreign nation.

“This was an FAA system that didn’t meet the moment,” he said. “Our contractor didn’t meet the moment or the equipment didn’t meet the moment and causes the outages.”

Duffy also warned that it would “take us time” to “resume normal operations”.

Among those affected by the fallout from the ground stop on Friday were sports teams traveling for games over the weekend. In college football, the Southeastern Conference announced that Mississippi’s game against Oklahoma had been rescheduled to Sunday – a day usually reserved for the National Football League (NFL).

Meanwhile, a men’s college basketball game between the University of North Carolina Greensboro and Cornell University, scheduled for Saturday, was postponed. NBA and NHL travel was also disrupted for the weekend.

On Saturday morning, there were reported flight disruptions at airports in major cities including Boston, Washington DC, New York and Miami.

American Airlines said in a statement on Saturday that “while we are taking every step to limit the impact on our customers and team members, the lingering effects of this major disruption to the national airspace system will continue today, including additional delays and cancellations”.

Delta Air Lines also warned passengers traveling on Saturday that they “should expect delays and additional schedule adjustments as we work to reposition flight crews and aircraft” after the FAA-ordered ground stop.

The UK’s Civil Aviation Authority (CAA) also issued a statement on Saturday morning warning that “while US flights have resumed, there may be some follow-on disruption as airlines work to re-book affected passengers and get all services back to normal”.

Friday marked the third nationwide ground stop issued by the FAA this century. The first occurred after the 11 September 2001 terrorist attacks. And the second took place in January 2023, during the Biden administration, after a pilot notification system went down.

During Friday’s briefing, Trump administration officials urged Congress – which until January is controlled by the White House’s Republican allies – to “fully fund a full build of a brand new air traffic control system”.

Meanwhile, on Saturday, New York’s governor, Kathy Hochul, a Democrat, called on Duffy to resign. She also called on Congress to “launch a full investigation to determine what went wrong and ensure it never happens again”.