ERBIL, Kurdistan Region of Iraq – US airspace reopened Friday evening after a nationwide ground stop lasting about four hours disrupted flights across the country, Transportation Secretary Sean Duffy said, blaming an outage caused by overloaded, outdated communications equipment.

"At 8:15 tonight Eastern Time, we've opened up the airspace," Duffy told a late-night White House briefing, adding that the process of returning flights to normal would be “going to be slow.”

More than 6,000 flights within, into, and out of the US were canceled during the ground stop, according to FlightAware. Disruptions continued into Saturday, with thousands more flights canceled.

Duffy said the outage occurred after the system’s switches and routers became overloaded with data. "Our switches, our routers actually got overloaded with data, and they went down," he said, adding, “We have old, old switches, old routers.”

He also said the US air traffic control system relies on “1980s little copper lines,” which went down “everywhere.”

The disruption came a day after Duffy and the White House formally asked Congress for $30 billion to upgrade and modernize the US air traffic control system. Duffy had made a similar request in late September, warning that further disruptions could occur without upgrades.

Meanwhile, Homeland Security Secretary Markwayne Mullin said there was no indication that the outage resulted from a deliberate attack.

“There is no indication as of yet that this was a nefarious attack against us, it’s just telecommunication outage as of right now,” Mullin said, dismissing speculation that a coordinated cyberattack had caused the disruption.