Air traffic across the country is resuming slowly after an FAA system outage briefly halted all U.S. flights this morning.

The Federal Aviation Administration lifted the ground stop around 8:50 a.m. ET Wednesday, adding the “cause of the initial problem” is being investigated.

According to the FAA, issues arose after an overnight outage in its Notice to Air Missions system that provides safety info to flight crews.

Known as NOTAM alerts, the system is crucial to planning flights and is used to share information about hazards in the air and on the ground, the New York Times reported.

NOTAM, a national system, is separate from the air traffic control systems that keep planes a safe distance from each other but is critical to air safety. Early reports indicate the issues were likely the result of a computer glitch and not something intentional but investigations continue.

The system was previously known as “Notice to Airmen,” a name that was changed in December 2021 to remove the gendered term and reflect the notices were also being sent to drone operators and not just pilots, CNN reported.

By mid-morning Wednesday, the FAA said operations were “resuming gradually” but delays could be expected as air traffic catches up from this morning. According to ABC News, this mornings stop delayed 3,700 flights and 640 cancellations.