Snipers don't use handguns.
To me, it is abundantly clear that if the shot did not come from the older guy who was initially dragged off, then it was a scoped rifle shooting from a much longer distance. Frankly, it is much more logical and practical to do that, as the distance involved would give the shooter at least a somewhat better chance of escaping. I would also hazard a guess that if a scoped rifle was used to assassinate him, the odds of a conspiracy go up manyfold.