A scary scenario, sir, and one featured in your novel.
beyond the realm of possibility? I think not. Sad to say, the thing seems all too likely, to me. And I'm not some manchurain candidate type either.
in a certain sense, these folks have not so much created these gunmen as they have created the circumstances under which they must inevitably arise.
urban poverty, insufficient law and order, anarchy in the family, on the block, and in the city as a matter of policy, these things must produce the super=predators who will do these things.
The bastards who will do this will do it for the pure joy of the thing, and not from any political motives.
The way that it often works is that the actual conspirators are puppet-masters who identify the group or individual who will pull the trigger or place the bomb, and enable them/him to do it. The conspirators provide cash, weapons if required, target information etc and simply allow the fringe group to do what they want to do anyway. After the operation, the conspirators fade back to invisibility, and may even burn their duped operatives.