Sean Duffy, the country’s transportation secretary, said his agency is ‘going to hold people accountable’.

The top United States transportation official has said work being performed on an antiquated telecommunications system could be to blame for a rare, hours-long US Federal Aviation Administration (FAA) ground stop that halted all air traffic inside the country.

At a news conference in Washington, DC, on Saturday, US Transportation Secretary Sean Duffy spoke about routine maintenance that was performed on Friday to upgrade a router from copper to fibre at a “very remote location”.

Duffy said “contractor error or a faulty piece of equipment” caused the router in question to start flooding other ones in the country’s air traffic control system.

“We had 1,000 connection attempts ever four to five seconds,” he told reporters at the Transportation Department.

When those connection issues caused disruption in radar feeds and communications between air traffic controllers and aeroplanes, Duffy said a ground stop was ordered.

“When we see that, your safety is paramount. We shut down the airspace,” he said.

The Transportation Department identified the contractor of Friday’s maintenance as L3Harris, a US-based aerospace and defence company that has been tasked with modernising the communications system for the FAA.

“What we’re working to determine was whether we had incompetence, meaning was it … installator error from L3Harris? Or did they install a faulty router?” Duffy said. “We haven’t determined what the fault was.”

Duffy added that L3Harris, which he said had performed decades of work for the FAA, “didn’t meet the moment”.

“They own this network. They’re our prime contractor. I can’t go somewhere else. They own it,” he said. “Or was it a faulty router? And we’re going to find that out. And frankly, we’re going to hold people accountable.”

Al Jazeera has reached out to L3Harris for comment on the allegations but has not yet heard back.

Amid concerns that Friday’s ground stop was the result of a cyberattack, the US government said Saturday it did not “believe” that was the cause.

“Once we were able to isolate the work being done at this location, all of our systems came back online,” Duffy said. “So, we don’t believe this was cyber at all.”

The head of the FAA, who also spoke at Saturday’s news conference, echoed the transportation secretary’s remarks, saying the system outage was caused by an equipment error.

“We see zero evidence of anything nefarious, and zero evidence of anything AI-related,” FAA Administrator Bryan Bedford said.

A root cause analysis would be shared with the public when it is available, he added.

As airlines and thousands of travellers begin the process of digging out from Friday’s groundstop, the longest and only second nationwide stop since 9/11 , the FAA said full schedule integrity is expected by the end of Sunday.

“We have crews and aircraft that are out of position for 12,000 flights that were lost,” Bedford said. “These crews, pilots and flight attendants need to get to their points of reconnection with their schedules.”

Bedford said the FAA expects about 4,000 to 5,000 flights out of the 41,000 scheduled on Saturday to be cancelled.

“That will set us up for a full recovery tomorrow,” he said. “I do think the worst is behind us.”