ROME, Oct. 16 -- Italy's airspace was running at about half its usual capacity on Friday morning after a fault in the data network that supports air traffic control, and the national air navigation provider could not yet say when full service would return.

ENAV, the provider, said the anomalies appeared overnight between Thursday and Friday. Its contingency systems kept traffic moving normally, with no loss of capacity, until 6 a.m. As traffic built and the faults persisted, it introduced flow-regulation measures that temporarily cut the number of aircraft Italian airspace can handle. By ENAV's own estimate, capacity was down by about 50 percent when it issued its statement on Friday morning.

The agency has ruled out a cyberattack or sa...