What to know about flights after the U.S. nationwide ground stop

Thousands of flights Saturday would likely be affected by delays or cancellations, but officials assured passengers that the nationwide ground stop was not caused by a nefarious actor and would not threaten ongoing safety.

The longest flight stoppage in 25 years ended around 8:15 p.m. ET Friday, allowing some domestic flights to take off from U.S. airports. Officials expected a full recovery by Sunday.

READ MORE: Thousands of flights canceled as airlines work to recover from nationwide ground stop

Secretary of Homeland Security Markwayne Mullin said there was no indication the failure was related to a cyberattack and that no lives were at risk.

Here's what to know.

Flights have resumed but the disruption continues

The system was stable Saturday after Friday's massive disruption, Federal Aviation Administration Administrator Bryan Bedford said.

Ultimately, 12,000 scheduled flights were unable to take off Friday, including 9,000 commercial flights. Bedford reminded passengers that airlines are obligated to offer free rebooking on another flight or a full refund if the flight was canceled. Passengers should contact their airlines for information.

WATCH: FAA issues rare nationwide ground stop due to 'extraordinary circumstances'

Of the 41,000 flights scheduled Saturday, as many as 5,000 will likely be canceled, Bedford said.

"That will set us up for a full recovery tomorrow," Bedford said. "I do think the worst is behind us, but we still do have a significant amount of recovery work to do."

Bedford offered an apology for stranded travelers and thanked air traffic controllers, pilots and flight attendants for managing the fallout.

The system was overloaded as a part of it was being upgraded

A contractor had been working on replacing a section of the system's old copper wire with new fiber wire Friday afternoon, leading to the system overload, Secretary of Transportation Sean Duffy said Saturday. Duffy said officials were investigating whether contractor error or a faulty router was the cause.

Officials stressed that the ground stop was not the result of a system outage or nefarious activity by a foreign actor. When they couldn't quickly locate the cause of the overload, they decided to keep flights grounded nationwide.

WATCH: Homeland Security Secretary Mullin says FAA flight stoppage 'isn't another 9/11'

"We did that out of an abundance of caution," Bedford said, saying they stand by the decision. "I'm sorry that we put you through this, but we did it for the right reasons."

The air traffic control modernization began last year after Congress approved $12.5 billion of funding. Duffy and the White House are pushing for $30 billion more to complete the work.

The Microsoft outage and cyberattacks were unrelated to the stoppage

Microsoft said it had no indication that the ground stop was related to the service disruption that took Outlook and other services offline Thursday, a spokesperson for the company said.

Microsoft said Saturday that services were fully restored after its massive outage.

READ MORE: Microsoft says services fully restored after global outage

Users reported that Microsoft 365 services, including Outlook, Teams and AI assistant Copilot, had been interrupted.

Over 430 million people use Microsoft 365 apps, Microsoft said on its website. Problems started appearing around midday Thursday in the U.S.

A free press is a cornerstone of a healthy democracy.

Support trusted journalism and civil dialogue.