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.

Transportation Secretary Sean Duffy said that "inconsistency" in data from routers in the air traffic control system led to the FAA issuing a nationwide ground stop Friday.

At 3:36 p.m, "we experienced instability for our radar and our communications in our centers and our TRACONs across the country," he said, referring to Terminal Radar Approach Control facilities.

Duffy described an "incredibly old air traffic control system" that uses analog telecom.

"Our switches, our routers, actually got overloaded with data ... and started to give us inconsistency in the data from those routers. And because of that, we shut the system down," he said.

Homeland Security Secretary Markwayne Mullin reiterated that there was "no cyberattack" at the top of a press briefing Friday on the ground stop.

"I just want to reiterate this: The public was never at danger through this process. There was never any lives at risk. There was no flights at risk," he said.

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

In an update shortly after 6 p.m. ET, the FAA said that the ground stop remains in effect.

"As enroute facilities recover, flights will be allowed to depart through coordination with the air traffic control system command center."

-ABC News Ayesha Ali