Boston BOmb

The explosion at the Boston Marathon is just sad. who a would want to do something like that and why would they? What if this was sent by another country? Why would anyone want another war?
People saw a mysterious man on the roof leaving after the explosion. Could he be the person that did it or a worker of someone? What did Boston do to get this? I heard a guy was suppose to propose to his girlfriend when crossed the finish line, but she died. How do you think that made people feel?