Flight are listed on a board inside a terminal at Seattle-Tacoma International Airport on Friday. Lindsey Wasson/AP hide caption

Lindsey Wasson/AP

The nationwide ground stop that paralyzed the U.S. aviation system on Friday has ended, according to the Federal Aviation Administration.

The rare stop, which lasted about four hours, ground all U.S. airports following a "communication issue," Transportation Secretary Sean Duffy said Friday afternoon.

At a press conference Friday night, Duffy said the issue appears to be old technology.

"We have 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.

Homeland Security Secretary Markwayne Mullin joined Duffy at the press conference and said there was no evidence the issue was due to an attack.

"We have seen zero evidence this is a cyber attack," he said.

As of 8 p.m.…