Widespread travel disruption is expected across the US this weekend as aviation recovers after all flights in the country were grounded on Friday afternoon.

The FAA halted flights nationwide for about four-and-a-half hours after an issue with air traffic control systems, stranding travelers in airports and canceling about 12,500 flights on Friday and Saturday.

Still, Friday's closure will snarl air travel into the weekend as airlines scramble to get airplanes and crew back in place — and thousands of delayed or canceled passengers back on their way.

Department of Transportation Secretary Sean Duffy and Homeland Security Secretary Markwayne Mullin, during press conferences on Friday and Saturday, blamed the outage on outdated ATC infrastructure. Duffy also floated the possibility of an error by longtime FAA contractor L3Harris Technologies, or a faulty router.

They also called for Congress to cough up $50 billion for ATC modernization — up from the $30 billion previously pitched in September. The FAA tapped AT&T in August 2026 to lead the infrastructure overhaul of L3Harris' decades-old network.

Here's what we know so far about the cause of the FAA outage.

L3Harris did not immediately respond to a request for comment from Business Insider.

Duffy floated a contractor mistake or a faulty router

Duffy said on Saturday that the issue began when L3Harris, an FAA contractor for over 20 years, was upgrading copper wiring to fiber at "at least one" remote FAA site on Friday; the fiber work has been ongoing for months across the US.

One of the routers in the project "started flooding other routers" in the network with data, so the FAA shut it down. That affected radar and communication with pilots at Air Route Traffic Control Centers, which manage and guide traffic at higher altitudes between airports.

"What we did yesterday was routine; this has happened thousands and thousands of times over the course of decades in America," Duffy said. "These issues don't happen, which begs the question for us: What did L3 Harris do? Did they make mistakes yesterday?"

"They own this network; they're our primary contractor. I can't go somewhere else; they own it," he added. "Or was it a faulty router?"

Duffy said Friday that the Trump Administration had "already replaced some of those switches" and they did not go down: "It's the old hardware that went down," he said.

The IP switches are new voice communication systems designed to replace the decades-old analog voice switches that much of the national airspace system still relies on.

Duffy and Mullin also emphasized both days that the fault wasn't related to cybersecurity or AI. Mullin had dubbed "SI" for the Trump Administration's preferred term, "superintelligence." Mullin said late Friday that the "public was never in danger" during the airspace outage.

Duffy said Friday they were still "working on the details" of exactly what happened — what caused it and how it could be prevented in the future. He said he couldn't guarantee it wouldn't happen again when asked if the traveling public should feel safe when flying in US airspace.

That flying public was stranded in airports around the country — checking airport monitors only to see their flights delayed or canceled. Because the events of the day weren't the airlines' fault, passengers would have little recourse for meals or hotels.

How the FAA's nationwide ground stop went down

Timeline of Friday's key events (all times ET)

• 3:36 p.m. — US air traffic control begins experiencing issues.
• 3:54 p.m. — FAA issues nationwide ground stop for all planes.
• Roughly 7:30 p.m. — Over 4,000 flights have been canceled.
• 8:15 p.m. — Nationwide ground stop lifted.
• 10:30 p.m. — Almost 9,500 flights across Friday and Saturday have been canceled.

About 20 minutes after the ground stop was ordered, Duffy said it came "following a communication issue impacting all Air Route Traffic Control Centers."

People were quick to question whether SMART, a new FAA AI tool to help air traffic control that started deploying this week, was involved.

ASI, the company that makes the SMART system for the FAA, said that Friday's issues weren't "based on or a result of the SMART system, any product of ASI or our integration with FAA and ATC."

Duffy also said SMART was not at fault.

Mullin said earlier Friday, when asked if the AI system might be related to the outage: "That's what we're looking at."

He also said that it had "no indication" the ground stop was related to Microsoft, which had faced massive issues across its services this week.

Meanwhile, the FAA had said the ground stop was due to "extraordinary circumstances and for reasons of safety."

Three hours after traffic was halted, more than 4,000 flights had been canceled or did not take off, according to data from aviation analytics firm Cirium.

By 10:30 p.m., that figure grew to nearly 9,500. As of early Saturday morning, that number remained the most up-to-date. That number includes flights scheduled to depart the US on Friday or Saturday.

Chicago O'Hare topped Cirium's list with 205 cancellations, followed by Atlanta's Hartsfield-Jackson — the world's busiest airport — with 195.

More than 100 flights were canceled at major airports in New York, Boston, San Francisco, Dallas, and Denver.

Homeland Security said FAA ground stop wasn't a safety issue

Mullin said the ground stop wasn't a safety issue. He called it a communications-related issue. He said he didn't want people to assume this is a safety threat to the American people or "another 9/11."

Still, many planes had to divert or turn around.

Among the flights affected was Delta Air Lines Flight 958, which was traveling from Los Angeles to New York's JFK. Almost four hours into its journey, at 4:11 p.m., the Boeing 767 turned around and diverted to Cincinnati.

Another Delta flight, from Atlanta to JFK, changed course over the Eastern Seaboard and diverted to Baltimore.

Pilots in the air have a system for these issues

Mark Stephens, a retired Delta Air Lines pilot, told Business Insider that pilots have a system for these situations to maintain safe distances from other aircraft and land within their limits.

Planes en route during a ground stop can still land, Stephens said, noting that they "continue via the flight plan, and then start using backup methods."

He said there are digital tools that can connect them with air-traffic control and their airline's dispatch to obtain routing or other information.

Stephens said that unless someone tells the pilot to divert, the plan is "stay the course and do what you're trained: aviation, navigate, communicate."

The last time a nationwide ground stop occurred was in January 2023, and it lasted only about 90 minutes. Before that, it was in 2001 on 9/11.

The sheer scale of the ground stop is likely to create a snowball effect into the weekend.

"Long delays will turn into a crew rest nightmare," Stephens said.

Saturday is "going to be a very difficult reset as pilots and flight attendants will be all over the country, and planes will be stuck in cities without crews," he continued, adding that "major hubs don't have space for all these planes."

Passengers are facing travel chaos

Some travelers were taking matters into their own hands.

Amtrak said on Friday evening that its website and app were generating error messages. "We're currently experiencing higher-than-normal call volumes due to ongoing airline issues," the train operator said in a post on X.

At airports around the country, passengers were facing crushing delays.

A passenger at the Springfield-Branson Airport in Missouri said he was told all flights were canceled for the rest of the day, and that he should rebook — but when he tried to do so, airline systems were overloaded, and he couldn't get through.

Airlines begin recovery, waive change fees

Late Friday, major US carriers said they were resuming operations after the FAA lifted the ground stop, but warned that delays and cancellations would continue as aircraft and crews were repositioned.

Delta said it was "safely resuming operations," but cautioned that it would take time to reset its network. Southwest said it would resume a limited number of flights after and focus on repositioning its planes.

United said it had resumed some flights Friday night, while JetBlue expected disruptions to extend into Saturday after canceling all of its remaining Friday flights, except Europe-bound flights from JFK and Boston.

American said it was prioritizing international flights and flights to non-hub cities Friday night to position planes and crews for a recovery on Saturday.

"For the remainder of the evening, we are prioritizing flights to non-hub cities, as well as international flying, to position aircraft and crew where they are needed to help restore our operation," the airline said in a statement to CBS News.

American, Delta, Southwest, and United issued travel waivers allowing eligible passengers to change or rebook flights under carrier-specific terms.

Noah Sheidlower and Alcynna Lloyd contributed to this report.