WI: The Boston Marathon bombs went off under the crowd stands

Say that for whatever reason (perhaps an incredible stroke of luck), the Tsarnaev brothers are able to hide the bags containing the bombs under the crowd stands in Boston. They promptly go off more or less the same time as IOTL.

Just... how high would the casualties be? What would be the aftermath of a worse Boston Bombing?

How bad could things possibly get?
 
The consequences would be so frightening-
& the death toll would have been so high-
that, to be honest, I don't even want to think
about them.
 
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