“Man previously carrying a black backpack has moved away from the scene prior to the black with out his backpack”
I think this label in the photo has the chronology backwards. I think he still has the backpack, but has set it on the ground at his feet.
There is a larger version of this photo here:
http://www.freerepublic.com/focus/news/3008859/posts?page=152#152
In the larger version, we can see the race clock (not real time, but race time) says 2:35. The bombing was at 4:09 race time.
There are other photos which show the crowd having thinned out and the two guys having moved up to where the bomb blast occurred. In these photos, the guy with the black backpack is seen with his backpack, so he probably also has it in your photo, but we just can’t see it.
In any event, LEO knows who these guys are and are checking them out.
Supposedly these guys were runners. The winning time was 2:10:23. Assuming these guys are really great runners, what was their chip time? Did they have time to finish their run, suit up, and start blending in with the crowd by 2:35 clock time as shown in your blown-up picture? I wonder if they can be picked out in pics prior to the winning time, demonstrating that they didn’t run the race at all. There should also be pics showing them crossing the finish line.