The U.S. Federal Aviation Administration said Wednesday normal air traffic operations are gradually resuming across the United States after a malfunction affected one of its computer systems, delaying 5,400 flights and cancelling another 900.

The FAA had earlier ordered airlines to pause all domestic departures for several hours due to an issue with its Notice to Air Missions System but now says the “ground stop has been lifted.” Military flights in the U.S. were not affected.

The White House Initially said that there was no evidence of a cyberattack. But President Joe Biden said “we don’t know” the cause of the computer outage. He told reporters he’s directed the Department of Transportation to investigate the cause of the disruption.

Biden early Wednesday said he had just been briefed by Transportation Secretary Pete Buttigieg, who told him they still had not identified what went wrong.

“I just spoke to Buttigieg. They don’t know what the cause is. But I was on the phone with him about 10 minutes,” Biden said. “I told him to report directly to me when they find out. Air traffic can still land safely, just not take off right now.

“We continue to look into the cause of the initial problem,” the FAA tweeted.

Update 5: 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

—The FAA✈️(@FAANews) January 11, 2023

“I have been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots,” Buttigieg tweeted after the problem was first reported.

Before taking off, pilots are required to consult NOTAMs, or Notice to Air Missions, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system once was telephone-based, with pilots calling dedicated flight service stations for the information but has now moved online.

Breakdowns in the NOTAM system appear to be rare.

“I don’t ever remember the NOTAM system going down like this. I’ve been flying 53 years,” John Cox, a former airline pilot and now an aviation-safety consultant, told The Associated Press.

According to FAA advisories, the NOTAM system first failed at 8:28 p.m. Eastern time on Tuesday, which prevented new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

Air Force spokeswoman Ann Stefanek said the U.S. military flights were not impacted because the military has its own NOTAMS system separate from the FAA system and the military’s system was not affected by the outage.

European flights into the U.S. appeared to be largely unaffected.

Geoff Freeman, president of the U.S. Travel Association, which represents all components of the travel industry, said in a statement, “Today’s FAA catastrophic system failure is a clear sign that America’s transportation network desperately needs significant upgrades.”

“Americans deserve an end-to-end travel experience that is seamless and secure,” he said. “And our nation’s economy depends on a best-in-class air travel system. We call on federal policymakers to modernize our vital air travel infrastructure to ensure our systems are able to meet demand safely and efficiently.”

Some information for this report came from The Associated Press.

•A Southwest Airlines passenger jet taking off from Chicago’s Midway Airport as flight delays stemming from a computer outage at the Federal Aviation Administration brought departures to a standstill across the U.S earlier, Jan. 11, 2023, in Chicago.