The Federal Aviation Administration announced that the nationwide ground stop it issued earlier on Friday has been lifted, nearly five hours after thousands of flights were canceled across the country.

Transportation Secretary Sean Duffy said in a press conference Friday night that the airspace has reopened and some flights have started to take off, but warned that "it'll be slow."

"It's going to take us some time to unwind what happened this afternoon," he said.

Duffy said that "instability" in the radar communications at air traffic control centers began just after 3:30 p.m. ET. At the time, there were more than 3,500 planes in the air when the ground stop was issued —he blamed it on an "incredibly old air traffic system."

"We have old switches, old routers, we use 1980s little copper lines," Duffy said. "It's an old system that went down. It went down everywhere. It got overloaded."

He w...