Exhibit B is the fact that the bomber or bombers unknown waited until all the VIPs that were there for the first-to cross-the-finish-line photo-op had safely gone on to the next photo-op.
I don’t read much into that fact except for that is when the security got let down, which then allowed the bombers to strike.
Security is for the Official People, donchaknow? Not for the average at paying shlub.