I saw elsewhere that the plane had made it to near its planned destination and circled round, flying back towards the southwest and MD-DC, was unresponsive, and was in restricted air space near the White house. Fighter jets were scrambled to intercept a potential threat. Some speculation the jet was shot down, unconfirmed. The other obvious theory is pilot incapacitation due to medical emergency such as heart attack or hypoxia.
Numerous region-wide reports of a sonic boom (from the fighter jets, presumably).