The Federal Aviation Administration ordered a nationwide ground stop Friday afternoon after a telecommunications outage affected multiple Air Route Traffic Control Centers, the facilities that guide aircraft at high altitudes along their routes.

The agency said it halted departures out of an "abundance of caution" and lifted the stop at 8:15 p.m. ET after more than four hours.

More than 3,500 airplanes were in the air when the order was issued, according to CNN, and the FAA said it thoroughly tested systems to confirm they were stable before clearing flights to resume.

Nationwide ground stops are extremely rare; this was only the second of its kind since the September 11, 2001 attacks, and the longest, per the BBC.

Officials attributed the failure to outdated equipment rather than any attack. Transportation Secretary Sean Duffy said telecommunications routers were overloaded with data and "went down."

Homeland Security Secretary Markwayne Mullin said there was no evidence of a cyberattack or a link to the prior day's Microsoft outage, saying it "appears this was mechanically driven" and that no one was ever at risk.

According to CNN, Mullin pointed to recent FAA updates as the trigger, saying new technology was added to an existing framework and overwhelmed the older system.

What makes it more alarming, the BBC reports, is that the last comparable nationwide stop came after 9/11 — and officials warned such failures could recur.

The disruption was enormous. CNBC, citing flight-data firm Cirium, reports more than 10,000 flights were disrupted, with about 11,000 flights scheduled to depart after 4 p.m.

ET carrying roughly 1.5 million passengers, and around 600 flights diverted. According to CNN, flight tracker FlightAware counted more than 15,000 US flights delayed or canceled on Friday, with about 3,000 more expected affected Saturday.

Flightradar24, cited by the BBC, puts cancellations through Saturday at nearly 10,000 — Delta and American each above 2,200, United at 1,800 and Southwest near 1,400.

Duffy acknowledged at a Friday press conference that recovery would be slow but said he expects the system to be back to capacity heading into Thanksgiving travel, which begins in less than two weeks.

Former TSA Administrator John Pistole told the BBC he is "not optimistic" about how quickly airports will recover and called the outage a national security concern.

The ripple effects reach beyond travel: Variety reports Hollywood's Governors Awards, the American Film Market's final days and the Rock and Roll Hall of Fame induction could face delays as talent and executives scramble during one of the industry's busiest stretches.

Airlines moved to reset operations even as chaos persisted. CNBC reports JetBlue told staff it would cancel the remainder of Friday's flights — except European routes — plus some Saturday flights, so crews and aircraft are positioned to resume.

Delta said it was safely resuming flights, United restarted some Friday-night flights, and JetBlue planned to fly again Saturday morning, per the BBC.

The BBC also notes inbound UK-US flights faced delays and cancellations at Heathrow, though Manchester Airport said its three US-bound Virgin Atlantic services were only slightly delayed.

The outage has reignited a funding fight. Duffy warned "it's going to continue to fail" without upgrades and urged Congress for more funding; the BBC reports he made a plea for $50 billion for a new "state-of-the-art" system, a figure Mullin also cited on Friday.

CNN notes Congress already approved $12.5 billion last year for an overhaul that has replaced copper wires with fiber optics and upgraded hundreds of radios, yet aviation safety expert David Soucie told CNN the systems' interdependence makes fixes complex.

Lawmakers on both sides, per the BBC, are calling for an investigation into what happened to ensure it never happens again.