The Notice to Air Missions (NOTAM) system, a centralized database operated by the US Federal Aviation Administration (FAA) that provides key safety information for all air travel nationwide, was back online Wednesday morning after an outage that grounded flights across the country.

NOTAM provides an automated, central source of critical flight data for aviators, noting information like closed runways at particular airports, equipment outages, and hazards along travel routes. The FAA's first public notice of the outage was in a tweet sent at approximately 6:30 a.m. Eastern Standard Time, but it's unclear at what point, overnight, that the system actually went down. As of 9 a.m. EST, the FAA said in a follow-up tweet, the system was fully restored.

Flights already in the air at the time of the outage were deemed safe to land, but the outage snarled air travel across the country, as the FAA implemented a ground stop at all airports in response. Newark and Atlanta both began to resume departures even earlier, the FAA said, "due to air traffic congestion in those areas." A spokesperson for Atlanta's Hartsfield-Jackson International Airport, said that 13,000 passengers locally, and 43,000 nationwide, had been affected by the ground stop, according to the New York Times.

An industry group, Airlines for America, warned that the ground stop would cause "significant operational delays" around the country, and the full impact of the NOTAM outage is still being assessed. Root causes of the outage are were still unclear shortly after flights resumed, though nothing, including a cyberattack, has yet been ruled out. US Transportation Secretary Pete Buttigieg's latest statement said that, while the system has been restored, the search for the direct cause of the outage is still under investigation.

As of 9:30 a.m. EST, data from FlightAware indicated that almost 4,600 flights in the US had been delayed, with another 832 cancelled. Flights from some airports were apparently still affected by local ground stops, including one at Chicago O'Hare, a critical air travel hub. United Airlines has pledged to waive change fees and rebookings for delayed and cancelled flights resulting from the outage.

"The likely consequence of this is obviously a series of delays and cancellations across the US with perhaps some impact into longer-haul international flights," said John Grant, chief analyst at OAG, a travel data analysis company.

The outage comes amid a push to modernize the NOTAM system and transform it into a modern, fully automated framework, which has been under way since 2018.