Trump administration officials said Friday that new technology being added to the nation’s air traffic control system overloaded antiquated parts of it and caused “intermittent” communications and radar gaps, leading to the longest nationwide ground stop in 25 years.

Axar.az reports, citing The Associated Press, that it was lifted in the evening after a five-hour stretch when most flights were not allowed to take off from airports in the U.S.

Thousands of flights and potentially over a million travelers were affected. Secretary of Homeland Security Markwayne Mullin told reporters at the White House that no one was in any danger as more than 3,000 flights were safely landed. But Secretary of Transportation Sean Duffy said he could not guarantee it wouldn’t happen again.

Mullin said the FAA was installing “new equipment, new technology” and “it was overwhelming the older system.” There appeared to be no indication of a cyberattack, he said.

A limited number of domestic flights began departing late Friday after the ground stop was lifted, but it will likely take days for airlines to fully recover because so many planes and crews were out of position. Officials said they think the system will be back to normal in time to handle the Thanksgiving travel rush.