Normal air traffic operations are “resuming gradually” after a mass system failure grounded commercial U.S. flights Wednesday morning, the Federal Aviation Administration (FAA) announced in an update.

In a fifth update Wednesday morning, the FAA announced normal air traffic operations are “resuming gradually across the U.S. following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews.”

“The ground stop has been lifted. We continue to look into the cause of the initial problem,” it added:

The update followed the news of the FAA working to restore the Notice to Air Missions System following what it described as an “outage.” As a result, the FAA initially ordered airlines to pause all domestic departures until 9 a.m. ET.

“All flights currently in the sky are safe to land,” the FAA wrote in an update Wednesday morning, noting that pilots check the NOTAM system prior to flight departure.

“A Notice to Air Missions alerts pilots about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight,” it added:

According to White House chief of staff Ron Klain, President Biden “directed DOT/FAA to restore the system quickly and safely, and to determine causes.”

“FAA has determined that the safety system affected by the overnight outage is fully restored, and the nationwide ground stop will be lifted effective immediately,” Buttgieg said in a statement Wednesday morning. “I have directed an after-action process to determine root causes and recommend next steps”:

The outage, causing the FAA to ground flights, follows the holiday chaos that consumed U.S. airports and airlines — particularly Southwest Airlines — after days of cancellations, leaving thousands of travelers stranded.

Buttigieg has deflected blame, however, stating during a recent appearance on Special Report that does not run Southwest’s system. However, he stated that the Department of Transportation “is using the full force of this department to make sure customers get taken care of.”

According to data from FlightAware, there were 6,139 U.S.-related delays and 984 U.S.-related cancellations as of 10:45 a.m. ET.