The Federal Aviation Administration issued a rare nationwide ground stop Friday afternoon, after outdated air traffic communications equipment became overloaded with data and began to fail, officials said.

The outage stranded thousands of passengers and caused widespread delays from which airlines and airports are still struggling to recover. It also highlighted persistent vulnerabilities in the aging U.S. air traffic infrastructure and a funding fight, as administration officials demanded lawmakers approve tens of billions of dollars more to accelerate systemwide upgrades.

Here’s what to know.

What caused the problem?

Sean Duffy, the transportation secretary, told reporters at the White House Friday night that the radar and other air traffic communications started to falter at 3:36 p.m. when old routers became overloaded with data. While the failures were not universal, they were pervasive enough to prompt officials to issue the unusual nationwide moratorium on takeoffs, Mr. Duffy said.

Appearing alongside him at the White House, Markwayne Mullin, the homeland security secretary, stressed that there was no evidence that nefarious activity had triggered the technology failures.

“We’ve seen zero evidence that this was a cyberattack,” Mr. Mullin said, suggesting that new equipment and new technology the Mr. Duffy’s department had added to the old infrastructure was what had overwhelmed it.

Mr. Duffy insisted that the failure was caused by the antiquated, legacy communications equipment, not recent upgrades to the system.