Anyways here's two guys the FBI would currently like to talk to...
Now that their pictures are out they will either be cleared if they come forward (like the other two above did) or at least IDed by somebody who knows them.
Also I believe the other two in the thread above were fingered out by some internet group a few days ago... and not really the FBI. Here's one of the sites. There were links to picures these groups were sharing and all four of these guys were in them. They have since removed or relabled pictures of the two cleared runners but there are still posts of pictures of these two guys. One is seen leaving a backpack on the ground close by the 8 year old that was killed.