I'm guessing accident, but if this were terrorism, the magnitude of the attack would only show how much of a joke they've become.
Aircraft was not in contact with ATC
What you said is what I first thought also. If this is terrorism, then we really have been kicking their ea***s in Iraq and Afghanistan if the quality of the people they get can only manange to get a small plane, giving up on breaching airport security, and ram it into an apartment building, doing minor damage compared to an airliner, not even being able to find the Empire State or Chrysler byuilding. That's a pretty pathetic terrorist attack. The evidence at this point indicates tragic accident; either the pilot had a heart attack or something, or it was a maintenance problem with the plane.