About mid afternoon in Boston, 15 April, two explosions in quick succession killed two people and wounded about twenty others.
With only initial information it appears these explosions were relatively small. The damage to windows nearby is not extensive and there is no significant blast seat. At this point it appears the explosive might have been a sugar chlorate mixture or a flash/black powder, judging by the smoke. The explosive was a low-velocity improvised device potentially packed with nails and other shrapnel. Almost anyone can build such devices and there is little stopping someone from getting into events like the Boston Marathon. So far no motive has been ascribed, nor any claim of responsibility.
About three or four additional improvised explosive devices have been found near the original site. According to news reports these are in the process of being dismantled and have not exploded.