U.S. air travel came to a halt around midnight when an outage in an FAA system led to a ground stop order for all U.S. flights until the incident was resolved. The FAA’s Notice to Air Missions (NOTAM) system failed, causing more than 500 flights to be grounded as technicians worked to restore the system.

The system was restored just before 9 a.m. ET Wednesday morning, but the root cause of the problem has not been released.

The White House said there was “no evidence of a cyberattack at this point,” and President Joe Biden directed the U.S. Department of Transportation to launch a full investigation to learn what may have caused the massive failure. The announcement was tweeted by White House press secretary Karine Jean-Pierre.

Following Jean-Pierre’s announcement, former NATO Supreme Allied Commander James Stavridis tweeted that whether or not the malfunctions were the result of a cyberattack, the incident served as a wake-up call to the possible consequences of a cyberattack in the future.

“The airline national stoppage may or may not be a cyber attack, but even if it is not, it certainly shows us what one could look like. Good wake up call,” Stavridis tweeted.

Microsoft describes a cyberattack as an attack aimed “to damage or gain control or access to important documents and systems within a business or personal computer network.”

The Associated Press called NOTAM “antiquated” and reported on how the outage showed how dependent air travel is on the system. Pilots must consult the NOTAM system online before flying.

Aviation safety expert and former pilot John Cox didn’t confirm if a cyberattack was possible or not, but he told the AP that in his 53 years of flying, he’s never heard the system “go down like this.”

“So something unusual happened,” Cox said in the AP article.

The NOTAM system is used to deliver important information such as plane accidents and provides real-time updates to pilots and other flight personnel regarding any abnormalities in the National Airspace System, Newsweek previously reported.

The outage comes only weeks after Southwest Airlines canceled thousands of flights, leaving many passengers stranded over Christmas weekend. Transportation Secretary Pete Buttigieg was highly critical of the Southwest Airlines issue, calling it a “system failure.”

People mocked Buttigieg on Twitter Wednesday morning, chiding his comments about Southwest Airlines after the FAA’s outage.

Newsweek reached out to Stavridis and the U.S. Department of Transportation for comment.