FAA is trying to determine what caused a system failure at a major US ATC center over the weekend, causing hundreds of flights to be delayed or cancelled.

The problem occurred at FAA’s center in Leesburg, Virginia, close to Washington Dulles International Airport. Washington National and Baltimore Washington International airports were the worst affected, with reportedly about 25% of flights in and out of those airports delayed or cancelled Saturday, Aug. 15. Dulles, New York JFK and New York LaGuardia were also affected.

Although the ATC system was restored late Saturday afternoon, delays continued through the weekend as airlines worked to reinstate service.

FAA tweeted that the problem was caused by “an FAA automation system that experienced problems” and then tweeted that “FAA is continuing root cause analysis to determine what caused the problem & is working closely w/airlines to minimize impacts to travelers.”

On Sunday, Aug. 16, the agency tweeted that all air traffic operations were back to normal, but gave no details on what had caused the system to fail.

Although no connection was made with the system failure, FAA at the end of April completed the replacement of its En Route Host computer system that managed traffic in US airspace. The new system, known as the En Route Automation Modernization (ERAM) system and provided by Lockheed Martin, now manages traffic in US airspace at all 20 of FAA’s en route ATC centers.