There is still scant information available on making a judgement on who was responsible for this.
Alex Jones, mouthpiece of the frothing at the mouth conspiracy theorist, claims the whole operations "stinks" of being a false flag operation - that this is US government inspired in an effort to devalue the Tea Party - or something.
It was the Boston Marathon, and the bombing was carried out on Patriots Day, a civic holiday in Massachusetts, and is to commemorate the first batles of the American Revolutionary War. Is the target and the date a coincidence, or is it an indication that this was the work of some extremist US libertarian individual or group?
Far too early to tell. Have to see what develops. Regardless, it was an appalling tragedy, and a vicious attack on civilians. Bombs packed with ball bearings are clearly aimed at inflicting the maximum damage to human life and limb.