I have a friend who runs marathons (especially the Boston one). She actually just missed the past summer olympics as far as qualifying. Thankfully she just got a new job and didn't have time to get ready for this race. Terrible terrible event to unfold. Hopefully the losses are minimal.
I think it is fairly obvious from reports on the scene that this was some sort of terror plot. Not going to speculate on who was behind this until further information comes out. seems like the explosions came from inside a hotel near the finish lines. reports a 3rd explosive device on a delay that was meant to target the first responders has been found.