If I had to guess, someone majorly dropped the ball ala the August 2001 warnings that terrorists wanted to fly planes into buildings. I'm picturing a Homeland style Claire Danes character who is utterly convinced that some major shit is about to go down at the Boston Marathon, and cannot get the biggest of the big wigs to sign off on cancelling the race. Instead, the brass decide to put security teams in place to be ready to stop anything that might happen, which would fit the body language of SEAL #1 (no hat) and SEAL #2 (hat) by the wall.
They are then pictured together after the bombs have gone off, and the thing that bothers me here is that they do not react to the carnage around them, but appear to be waiting for instructions as if they are trained.
Meanwhile, there's another group shot of three other guys, via infowars, who are reacting to the first bomb going off.
And one of those three guys is later seen using a radiological measurement device.
I don't know what to think. And I don't think we'll get answers to these questions, unfortunately. It's possible that this was another massive intelligence failure, and TPTB are desperately trying to save face and erase any evidence that they had credible information there may be an attack at the event. I say this because of all the info about bomb-sniffing dogs and so forth, but that may be a routine procedure every year at the Marathon. Perhaps Boston natives could clarify that?