..A guy watching the race from a rooftop? Or bigfoot. But seriously, no need for conspiracy theories now... I don't see how someone on a roof can aid a bombing in any way.
The possibilities are endless, he could be the culprit who planted it in the first place and is just stealthily watching the situation unfold, or he could also just be a man watching on the roof. Who knows? But, what I do know is that you can't pin a crime like this on somebody who was just seen on a roof, it's pretty suspicious but no.
Its extremely common in Boston for people to climb rooftops to watch events.
And yes it is quite common, but I find it quite unusual that he's the only spectator seen on a roof.