The answer is:
Holder's letter reads: "It has come to my attention that you have now asked an additional question: 'Does the President have the authority to use a weaponized drone to kill an American not engaged in combat on American soil.' The answer to that question is no."
What bothers me about that is how they might define "Engaged in combat". That could be made to mean somebody who has the POTENTIAL to do physical harm. I wouldn't trust osama if he were stripped naked and locked in a padded cell. I just don't see this answer being an iron clad guarantee that the govt couldn't use a drone to take out somebody who is "stockpiling weapons", i.e. a handloader who has 10,000 primers and 20 rifles.
I am your side brother. But there is no way in hell they are going to be able to Drone all of us at the same time. Plus, I doubt their is enough military personnel flying the drones that would actually follow an order to smoke an American. My son is in the Air Force and he would walk away from such a command GUARANTEED.