NEW YORK (AP) — The world’s largest aircraft fleet was grounded for hours by a cascading outage in a government system that delayed or cancelled thousands of flights across the U.S. on Wednesday.

The White House initially said that there was no evidence of a cyberattack behind the outage that ruined travel plans for millions of passengers. President Joe Biden said Wednesday morning that he’s directed the Department of Transportation to investigate.

Whatever the cause, the outage revealed how dependent the world’s largest economy is on air travel, and how dependent air travel is on an antiquated computer system called the Notice to Air Missions, or NOTAM.

Before commencing a flight, pilots are required to consult NOTAMs, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has moved online.

The NOTAM system broke down late Tuesday, leading to more than 1,000 flight cancellations and 7,000 were delayed flights by midday Wednesday, according to the flight tracking website FlightAware.

The chaos is expected to grow as backups compound. More than 21,000 flights were scheduled to take off in the U.S. today, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Airports in Chicago, Los Angeles, New York and Atlanta were seeing between 30% and 40% of flights delayed.

“We are going to see the ripple effects from that, this morning’s delays through the system during the day,” said Transportation Secretary Pete Buttigieg in an interview on CNN. “Now we have to understand how this could have happened in the first place. Why the usual redundancies that would stop it from being that disruptive did not stop it from being disruptive this time.”

Longtime aviation insiders could not recall an outage of such magnitude caused by a technology breakdown. Some compared it to the nationwide shutdown of airspace after the terror attacks of September 2001.

“Periodically there have been local issues here or there, but this is pretty significant historically,” said Tim Campbell, a former senior vice president of air operations at American Airlines and now a consultant in Minneapolis.

Campbell said there has long been concern about the Federal Aviation Administration’s technology, and not just the NOTAM system.

“So much of their systems are old mainframe systems that are generally reliable but they are out of date,” he said.

John Cox, a former airline pilot and aviation safety expert, said there has been talk in the aviation industry for years about trying to modernize the NOTAM system, but he did not know the age of the servers that the FAA uses.

He couldn’t say whether a cyberattack was possible.

“I’ve been flying 53 years. I’ve never heard the system go down like this,” Cox said. “So something unusual happened.”

According to FAA advisories, the NOTAM system failed at 8:28 p.m. Eastern on Tuesday preventing new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

The FAA ordered all departing flights grounded early Wednesday morning, affecting all passenger and shipping flights.

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When Molly Flaherty walked up to her home in Philadelphia on Christmas night, she was shocked to find four people huddled around her lockbox at the steps to her front door.

“It was 10 o’clock at night, so I was a little scared at first,” Flaherty told Insider. “Were these people that were trying to break into my house? Or were they at the wrong house? I even thought was there something wrong with my house? Was there gas or smoke or something?”

The group seemed more confused than anything, she said, so she approached them and introduced herself as the homeowner.

Nicole Brunet, who was standing at the lockbox, told Flaherty she’d rented the property on Airbnb for her parents, who were visiting for the holidays from California, Brunet told Insider.

But Flaherty had never listed her home on the vacation-rental website.

The listing was real, but Flaherty didn’t make it

Brunet said she initially thought she might have the wrong address, and Flaherty assumed the same.

But when Brunet pulled up the Airbnb listing, Flaherty said the address and photos matched her house.

Flaherty shared screengrabs of the listing — which has since been removed from Airbnb — with Insider. She said the images of the home’s interior were taken before she bought it in 2020.

After Flaherty insisted she’d never listed her house as a vacation rental on Airbnb, Brunet realized she’d been scammed, Brunet said.

It’s what Airbnb calls an “account takeover,” when a scammer steals an Airbnb owner’s password.

An Airbnb representative told Insider that the listing for Flaherty’s home technically wasn’t fake but was an outdated listing that was likely reactivated by a scammer.

Airbnb

The screengrabs showed that the listing had a 4.7-star rating and more than 100 reviews when Brunet reserved it.

“I’m a tech-savvy person,” Brunet said. “I read reviews. I looked at the pictures. I walked by the house before I rented it to make sure my parents would be happy there.

“That’s the craziest part about this whole thing,” she continued, adding that the listing “had every component of a real account.” The experience, she said, has “shattered” her trust in Airbnb.

In a statement sent to Insider, the company said that “issues like this are rare, and we were disappointed to learn about it.” It added, “We provided support to the guests at the time to help make things right, including a full refund, and have deactivated the listing.”

Brunet said that when she reported the listing to Airbnb that night, the company offered to book them another Airbnb. But she said there was “no way” she was going to do that since “there was no way for me to tell if they were real or fake.”