Well you beat me on the actor angle T. :-) I have a different article with possibly other actors. The most striking picture in the ones you posted was the first for me. There would absolutely be a trail of blood on the pavement, even if it was just a little bit. But I can't even see one drop. That is impossible. And the guy never even shows any pain at all. And then when you consider what the guy in the article said about bleed out and so on. And them leaving an amputee behind for the lady next to him. No frigging way! They would never do that. Even the dangling leg parts look fake.
Here is another picture showing possible actors. It's not as convincing as the one you posted I don't think, but possible actors still.
The cops are from Sandy Hook. Are they the same ones that are at Boston? It looks pretty close to me. Can't say positively though.