(Just to confuse things further) White hat/blue man are seen at bottom of photo in center, still walking together headed to other bomb area. There absolutely MUST be at LEAST two people involved in this attack. HAS to be.
I really think that those two guys look suspicious and they are doing whatever it is that they are doing right at the blast zone. The guy bending over is handling something that looks just like the picture of the pack that the FBI released. The other guy looks like he has a pistol in his pocket. At least to me, that's what it looks like.