I’d wonder if there were any significance to the time and just who was running the marathon, maybe passing the bomb site(s) when the explosions were set off, a somebody or group of them that might be a target of interest to terrorists.
The retail storefronts in close proximity might be of interest too. Who would be most likely to see explosive devices being planted? Someone planting bombs might be inclined to do it with friendly witnesses.
It may be that the whole thing was intended to maximize visibility, death, injury and fear, since the Boston Marathon is a noteworthy event, with deaths and injuries being random. But, maybe not.
Not to make light of a terrible situation, but I couldn't help laughing at a reader's comment on CNN's website earlier today where he/she made that exact point, but then noted that planting bombs at the finish line of a marathon would end up killing and injuring more Ethiopians and Kenyans than anyone else.