Following the lifting of the ground stop, United Airlines said it is "resuming operations, including some planned Friday night flights."

Transportation Secretary Sean Duffy called for funding from Congress to upgrade what he called an "incredibly old air traffic control system" in the wake of Friday's nationwide ground stop.

"This is an emergency," he told reporters. "I want Congress to give us emergency funding. This is bipartisan. This is American. I want $50 billion."

In describing Friday's issue, Duffy pointed to "old switches, old routers."

"We use 1980s little copper lines. It's an old system that went down and went down everywhere. It got overloaded," he said.

"I think our fix gives us a pretty good indication that it was a legacy network problem that went down," he continued, adding, "By the way, all of the new equipment that we put in, none of that went down."

On how long to expect delays and cancellations due to the ground stop, Transportation Secretary Sean Duffy told reporters, "I believe that it's going to take us some time to get back to capacity."

He said, "it's going to take us some time to unwind what's happened this afternoon," adding that "hopefully tomorrow we are going to start to see far better movement."

Duffy said he believes things will be "back up and operational" heading into Thanksgiving travel.

Pressed on whether the administration can say with 100% certainty this wasn't a cyberattack, Homeland Security Secretary Markwayne Mullin responded, "There's nothing completely certain right now," then said, "It doesn't appear that way."

"There is nothing that indicates it yet," he continued, saying there hasn't been any evidence of an entity claiming to be behind one.

"It appears that this was mechanically driven. However, we're still looking, so we're not going to rule that out," Mullin said.

The FAA said the nationwide ground stop is over, after more than four hours.