Sirius Black's death discussion?

For Harry Potter fans. Did Bellatrix stupefy him and the fall through the veil killed him or he was hit by a killing curse, Avada Kedavra, and then fell through the veil? What do you think? In the book it is only mentioned that the PREVIOUS curse Bellatrix directed at him was a jet of red light. That's all we know for sure. Also, this one is for Sirius Black's fans :) We know that after a person dies, a certain kind of spells they did can be undone - if Sirius was indeed stupefied and fell through the veil, he could have been held in there, perhaps even alive, and then when Bellatrix was killed by Molly the spell that kept him there was undone. So he COULD be alive. But it is not mentioned in the books because Harry finds out about it after the end of the book. (I don't really believe in this, but i really, really like Sirius so this would be an ideal solution in my opinion) :)