I was just thinking that the random attack seems less and less likely because if the police thought this was a possibility, wouldn't they have to warn people?
I think it is a layered, planned attack. Carried out by the sort of people I have had nightmares about and they are experienced and know how to do this stuff without leaving a trace. They have been engaged by someone who wants Sam gone for whatever reason ... the possibilities are endless but there is some reason they want Sam gone. They are possibly also being influenced by another layer of involvement ...
... some people look and are easily led ... it must be a big reason because you don't get someone knocked off because their voice annoys you or whatever but remember this stuff can build up over years, decades sometimes.