Okay, I’ve been doing some more thinking about this. At this point, I find it harder to believe that there just happened to be a drill going on coincident with the Boston Marathon than to believe that the claim of there being a drill was an excuse for the added (and obvious) police presence. While I think that’s a poor excuse, given how many people reflexively believe whatever police tell them, it seems quite likely to me to work nonetheless.
That being said, I’d still like Aristophanes to provide his source(s) for his claims of knowledge (as opposed to statements of belief) in this context.