The airline industry has slowly begun to resume service after a Federal Aviation Administration system outage caused thousands of flight delays and cancellations across the United States Wednesday.

The FAA briefly halted all domestic flight departures across the United States Wednesday morning, lifting the ground stop around 9 a.m. ET after it restored a system that provides pilots with pre-flight safety notices.

But airlines continued to delay or cancel flights because of ongoing congestion…

The affected system, Notice to Air Missions (NOTAM), sends alerts to pilots to let them know of conditions that could affect the safety of their flights. It is separate from the air traffic control system that keeps planes a safe distance from each other, but it’s another critical tool for air safety.