The US Federal Aviation Administration has lifted the longest nationwide ground stop of flights since the 11 September 2001 terrorist attacks.

The FAA said Friday's stoppage, which lasted more than four hours, was "due to extraordinary circumstances" from a communication issue affecting all air traffic control centres.

Homeland Security Secretary Markwayne Mullin said there was no indication of an attack and that everybody was safe.

Here's what we know.

The ground stop led to more than 8,500 delayed and nearly 8,000 cancelled flights on Friday, according to FlightAware.

The FAA's initial announcement at 15:54 EST (20:54 GMT) said all flights departing within the US would be grounded until further notice.

More than 3,500 airplanes were in the air at the time. Some were diverted to other airports and others flew on to their destinations.

Hours later, some flights were allowed to take off from San Francisco International airport.

The Federal Aviation Administration opened skies again at until 20:15 EST.

All US-bound air traffic from Eurocontrol member states was suspended "on request of the FAA due to technical issues", according to Eurocontrol's website.

There are 42 Eurocontrol member states, including dozens of EU nations, the UK, Ukraine, and Turkey, as well as Israel and Morocco.

Australia's largest international airport, Sydney, grounded all US-bound flights until further notice.

Air Canada cancelled flights between the US and Canada.

Transportation Secretary Sean Duffy said in a news conference on Friday night that the outage would cause a ripple effect for a few days.

"It's going to take us some time to unwind what's happened this afternoon," Duffy said.

Tens of thousands of people were stranded in airports around the country, many of them "without any knowledge of where they're going to sleep tonight", Duffy said.

Delta Air Lines and United Airlines said they were resuming operations.

American Airlines said it was "focused on operating as many remaining flights as possible this evening". JetBlue said it would resume flights on Saturday morning.

Duffy said a legacy air traffic control communications system became overloaded.

"So this is analogue telecom. Our switches, our routers actually got overloaded with data and they went down," Duffy said.

He said this had affected communications and radar. The system needs a complete overhaul, he said, estimating a $50 billion price tag.

"It's going to continue to fail," he said.

Homeland Security Secretary Markwayne Mullin told the news conference said they had seen "zero evidence" it was a cyber-attack.

He also ruled out any link to the global Microsoft outage that began on Thursday.

Mullin said the introduction of some new technology into an old air traffic control system had overwhelmed it.

Both cabinet secretaries insisted the public was never in danger during the stoppage.

It was only the second such nationwide stoppage since the immediate aftermath of the al-Qaeda plane hijackings of 11 September 2001.

"This looks like the worst since 9/11, but clearly not on that scale," Dr Guy Gratton, an associate professor of aviation at Cranfield University, told BBC Verify.

Dr Gratton pointed to a US grounding in 2023 due to a failure in the Notice to Air Missions (Notam) system.

"That lasted 1 hour and 52 minutes, delayed about 9,500 flights and cancelled about 1,300," he said.

The 2023 outage meant air traffic safety notices could no longer be published, and flights were grounded until that system was restored.

But Friday's incident lasted longer than the 2023 event.