It's starting to look like an individual did this - and that he/she was going for legs. Hoping to get legs of runners. This seems in line with someone enraged because they didn't qualify. Or hadn't qualified multiple times. So they destroy the legs of other runners who made it.
Just a theory.
http://www.nytimes.com/...
Boston Bombs Were Loaded to Maim
By KATHARINE Q. SEELYE, ERIC SCHMITT and SCOTT SHANE
BOSTON — The explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers, except they were rigged to shoot sharp bits of shrapnel into anyone within reach of their blast and maim them severely, law enforcement officials said Tuesday.
The pressure cookers were filled with nails, ball bearings and black powder, and the devices were triggered by “kitchen-type” egg timers, one official said.
The resulting explosions sent metal tearing through skin and muscle, destroying the lower limbs of some victims who had only shreds of tissue holding parts of their legs together when they arrived at the emergency room of Massachusetts General Hospital, doctors there said.
The scale of the attack and the crude nature of the explosives, coupled with the lack of anyone claiming to have been the perpetrator, suggested to experts that the attacker could be an individual or a small group rather than an established terrorist organization.
“This could have been a one-person job,” said Bruce Hoffman, a terrorism specialist at Georgetown University. “That makes it much harder to track. When we catch terrorists, it’s usually because they’re part of a conspiracy and they’re communicating with one another.”