Yes I just noticed in #56 that they have half the grate that goes around the tree pulled up and it is sitting behind the guy with the yellow boots so the bomb may well have been under the grate. I would think that would have greatly suppressed the damage of the bomb but also explains the horrific leg injuries if it was below people standing over it.
Furthermore the Fox 5 pictures, reportedly crime photos from the FIRST bomb also show shrapnel/bb’s imbedded into a grate. I therefore theorize that the bombs were placed well before the race under grates and police need to look for who was in the area in the nights before the race happened.