Duffy said a legacy air traffic control communications system became overloaded.
“So this is analogue telecom. Our switches, our routers actually got overloaded with data and they went down,” Duffy said.
He said this had affected communications and radar. The system needs a complete overhaul, he said, estimating a $50 billion price tag.
“It’s going to continue to fail,” he said.
Homeland Security Secretary Markwayne Mullin told the news conference said they had seen “zero evidence” it was a cyber-attack.
He also ruled out any link to the global Microsoft outage that began on Thursday.
Mullin said the introduction of some new technology into an old air traffic control system had overwhelmed it.
Both cabinet secretaries insisted the public was never in danger during the stoppage.