Boston, our hearts go out to you. If you have a loved one that was running in the marathon or attending the marathon, here is a Google people finder to help you find them. If you have information on people, you can also use the people finder.
Additionally, there is up to date info here: Live stream.
I am not going to dissect the situation and speculate about blame here. Of course, Wolf Blitzer and the rest of the left wing media speculated immediately that it was a right winger against taxes.
If you remember, any shooting over the past two years has been met with the same speculation. The facts show that the person who carried out the attack was either crazy, or a far left wingnut wanting to over throw the US government. Must be because they learned the lessons of the Port Huron Statement really well in school instead of things like Common Sense and de Toqueville.
On a side note, if you are running a large event like the marathon, Gogette Communications is a Chicago startup that can help you put everyone on the same communications page. They can tie up VHF, UHF and cell phones all on one common circuit so first responders can communicate to event personnel seamlessly.