The DHS secretary said there were no indications of a cyberattack.
The Federal Aviation Administration ordered a rare nationwide ground stop for all airports Friday -- a more than four-hour disruption that snarled travel from coast to coast with canceled flights and stranded passengers.
At a White House news conference Friday night, Transportation Secretary Sean Duffy blamed old technology that became overloaded and called on Congress to provide billions more in funding to modernize critical equipment, calling the situation an "emergency."
Department of Homeland Security Secretary Markwayne Mullin told reporters there was no indication the incident stemmed from a cyberattack, but that officials were still investigating.
Thousands of flights were canceled amid the ground stop.
"I believe that it's going to take us some time to get back to capacity," Duffy said when asked how long air travelers should expect delays and cancellations to continue.
12,506 flights have been cancelled so far between yesterday and today, per Flight Aware. The number is expected to grow throughout the day.
On Saturday morning already, 3,374 flights have been canceled and 1,008 are delayed nationwide, per FlightAware.
The total for Friday is 9,132 cancellations and 7,030 delays.
-ABC News' Clara McMichael
At a White House news conference, Transportation Secretary Sean Duffy insisted that the U.S. has the "safest airspace in the world" but also suggested an incident like Friday's ground stop would happen again and that flying will continue being “less safe” unless Congress provides funding for flight system advancement.
"We didn't think we would have this kind of outage," Duffy said. "The systems that were deployed kept everybody safe -- everyone landed. We're able to review what went wrong and then reboot the system and go. But you will have more of these. You will have more of them unless the American people and the Congress tell us to build."
Duffy called on Congress to provide $50 billion for a "complete overhaul" of the system.
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."
