http://www.zerohedge.com/news/2013-04-15/large-explosion-reported-boston-marathon-coply-square
The link above gives more details on what is happening. There were two explosions, worse still they happened in a very close interval, and from the pictures I've seen...
Ugly, very ugly.
They showed a picture from across the river and the smoke cloud from the bombing, it was like a mini 9-11. It's a beautiful clear day and this smoke cloud goes by.