Airlines canceled thousands more flights Saturday as they worked to recover from a nationwide ground stop that halted departures across the U.S. for more than four hours Friday, the longest since the Sept. 11, 2001, attacks.

More than 3,800 U.S. flights were canceled Saturday and more than 2,300 were delayed as of 10:08 a.m. ET, according to FlightAware. That's on top of more than 9,100 cancellations Friday.

The Federal Aviation Administration lifted the ground stop at 8:15 p.m. ET Friday. Transportation Secretary Sean Duffy said in a press conference Friday night that the airspace had reopened, 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 at 3:36 p.m. ET. More than 3,500 planes were in the air when the ground stop was issued. He blamed it...