Dead reckoning to blame?
A pilot that had flown with this man previously, has said his habit of flying by landmark (dead reckoning) and setting the auto-pilot, may be to blame for the impact. After clearing the mountains, this pilot had a habit of pointing it at the highest building (something he could see from quite a distance) and setting the auto-pilot.The speculation is ,as above, that he either had a heart attack from the stress involved, or was overcome by smoke in the cabin. The auto-pilot did its job.