Transportation Secretary Sean Duffy on Friday night asked Congress for $50 billion to build a brand-new air traffic control system. He spoke at the White House after a nationwide ground stop kept flights on the ground for nearly four hours.

Mr. Duffy said the trouble began at 3:36 p.m. Eastern time. That is when radar and communications became unstable at air traffic centers across the country. He said the Federal Aviation Administration stopped flights out of caution to keep travelers safe.

He said more than 3,500 planes were in the air when the stop began. He said the planes were brought in to land safely using the agency’s well-known steps for these events. The ground stop was lifted at about 8:15 p.m. Eastern time.

Mr. Duffy said old switches and routers became overloaded with data and began sending mixed-up information. “What we have is an incredibly old air traffic control system,” he said.

He said the new equipment already put in under President Trump held up. “I have already replaced some of those switches with President Trump — and our new IP switches did not go down. It’s the old hardware that went down,” he said.

Homeland Security Secretary Markwayne Mullin said officials have seen no evidence of a cyberattack. He said the failure appeared to be mechanical, though officials were still looking into it.

“This is an emergency,” Mr. Duffy said. “I want Congress to give us emergency funding. This is bipartisan; this is American. I want $50 billion. President Trump is a builder; he’s going to build this network and deliver for the American people, but the President can’t deliver for America unless the Congress delivers the money to make it happen.”

Mr. Mullin said the $50 billion figure is the administration’s estimate for a complete overhaul of the system. “We need someone to come in and build the whole system,” he said. “Nickel and diming it together does not work.”

The new request is up from the $30.1 billion that Mr. Duffy and White House budget director Russell Vought asked Congress for in a letter earlier this week.

Mr. Duffy said President Trump already secured $12.5 billion toward the rebuild through the One Big Beautiful Bill. He said that money is being spent well, but Congress knew at the time it would not be enough. He said lawmakers now have a chance to provide the rest in an emergency funding bill before the end of the year.