David Johnson had his cross-country flight delayed twice. Barbara Steinberg feared she would miss her high school reunion, while Grissel Francis could not shake memories of the Sept. 11, 2001, hijackings.

"It was kind of scary," Francis said Friday, as the United States experienced its largest disruption to air travel since the 2001 attacks. "You have these flashbacks from 9/11, you know."

They were among tens of thousands of passengers whose travel plans were thrown into disarray after the Federal Aviation Administration (FAA) imposed a nationwide ground stop that lasted four hours. The disruption resulted in more than 6,600 flight cancellations and 8,600 delays, according to FlightRadar24.

It was only the third nationwide ground stop issued by the FAA this century.

Transportation Secretary Sean Duffy said the disruption was not caused by malicious activity but by a failure involving aging network technology in the radar and communications systems used to manage air traffic.

At Ronald Reagan Washington National Airport, information displays were almost completely filled with flights marked "Delayed" or "Canceled." Some travelers appeared resigned as they wondered how long the disruptions would last and how their travel plans would be affected.

The uncertainty frustrated Johnson and other passengers.

"I don't know at this point how something like this could be happening and how this situation can last so long," said Johnson, who was trying to fly from Washington to San Francisco. "I could make alternative plans if at least the duration (of the delay) was known and clearly communicated."

Steinberg doubted she would make her reunion after her Porter Airlines flight from Newark, New Jersey, to Toronto was canceled, forcing her to rebook for Saturday.

"There's a lot of infrastructure that needs to be fixed, and that's what somebody needs to figure out," said Steinberg, an accountant from Cedar Knolls, New Jersey.

Debra Bliesner, 71, of Corralitos, California, viewed the FAA's ground stop as a prudent decision.

"I spent 30 years in the Air Force, so when the FAA is issuing a stoppage, I take that pretty seriously," said Bliesner, who was at San Jose Mineta International Airport for a flight to Palm Springs. "So, my prayer on the way in here was, if my safety is compromised ... I mean, I'd rather be inconvenienced," she said.

Grissel and Joseph Francis were already aboard a flight from Dallas to New York when the crew announced that all flights across the country had been grounded and that their plane would be among the last to reach its intended destination.

"It was just a surreal moment," Grissel Francis said.

After landing at New York City's LaGuardia Airport, their plane waited on the crowded tarmac for about an hour for an available gate, alongside several other aircraft lined up behind it.

Even after the FAA lifted the ground stop, tens of thousands of travelers remained stranded at airports, possibly overnight, Duffy said.

Edward Karagbo, of West Chester, Pennsylvania, was scheduled to take an Ethiopian Airlines flight Friday to Lome, Togo. Instead, he found himself at Newark Liberty International Airport waiting for a plane that had been rerouted and rescheduled to depart nine hours later.

"I have to squat the night here," he said. "I'm stranded."