There's not a lot of solid information yet so what follows is speculative.
I'm watching MSNBC while type this. They are now confirming that two bombs blew up at different locations along the marathon route.
First observation: If this turns out to be an act of terrorism, I'd say odds are that it's an act of domestic terror. It's no secret that the intelligence agencies have been focused on the threat of foreign terrorism and have been successful in thwarting a number of such. OTOH, the threat of domestic terrorism has received far less attention and resources, except where such supposed threats could be associated with protests like those mounted by OWS and the Occupy movement.
Second observation: This bombing is very reminiscent of the Atlanta Olympics Bombing. In that instance the bomb was likewise placed in a public space with the explicit purpose of targeting random individuals attending a public event. Like the Atlanta Olympics, the Boston Marathon is very much an international event with participants and spectators attending from around the world.
I could well be wrong but my gut is telling me that the authorities need to be looking for a homegrown perp.
1:45 PM PT: MSNBC is now reporting that a third device was found. Where it was found and why it didn't explode could be significant. It may have been a secondary bomb intended to target first responders and police. Eric Rudolph, the Atlanta Olympics Bomber, used such secondary devices in subsequent bombings.
2:06 PM PT: The police have announced another bombing at the JFK Library.
2:35 PM PT: Now they are retracting the report of the bombing at the JFK Library. Those of you who think it's too early to draw any inferences can feel a degree of vindication.
3:00 PM PT: Boston Police say they have no suspect in custody.
3:26 PM PT: I didn't want to go too far out on a limb earlier but it seems my circumspection was ill considered. Let me add that the particular scenario we've seen so horrifically played out in Boston today, a series of sidewalk bombs set off in crowds gathered for a public event, is one that has been promoted in white supremacist publications in this country. Most people would not be aware of this. Just as most people were unaware of the parallels between the Oklahoma City Bombing and the racist, terrorist fantasies of The Turner Diaries until the connection came out in court proceedings.
Again this doesn't prove anything and I could well be wrong. However, considering that the RW is rushing to blame it on Islamic terrorism, just as they did in the case of Oklahoma City, we'd do well to emphasize that there is just as much reason, if not more, to suspect domestic terrorism.
3:47 PM PT: MSNBC Is reporting that as many as 6 devices are being investigated. It's not clear if this is six in addition to the two that exploded or whether this includes them.
4:09 PM PT: One of the dead is reportedly an 8 year old child. I think I'm going to go have good cry.