I'm a long-time runner, 9-times Boston, not there yesterday but two training partners were.
The sane posters on this board have it exactly right: it's a wide-open event stretching 26 miles on a rather narrow road that passes through community after community and ending in a huge metropolitan area. The concept of sealed-off is just silly.
Everything and anything that happens now seems to be grist for the NRA/Tea Party/nut case mill.
All that is needed to make a happy life is within yourself, in your way of thinking. -Marcus Aurelius