Actually, this person has been referred to as an assassin since long ago. I remember it being discussed in John McAdams newsgroup. They can't tell the difference between a twig and a rifle, and no assassin is going to hang around out in the open putting his rifle on display for everyone over a minute after the assassination took place.
Considering the minuscule amount it takes for some people to whisk up a theory, I was surprised I hadn't seen this come up before.
I think James has provided the answer why it didn't take off in an earlier post.