Above is the Hezbollah operative trying to look like a dorky tourist waiting for Israelis to get on to a bus in Burgas, Bulgaria, into which he would then place his heavy black backpack containing a bomb which blew up, apparently ahead of schedule, since he was killed in the explosion.
Now in the case of Boston we're again looking at baseball capped young men looking like part of the crowd but upon reflection with overly large backpacks and looking a bit too much like trying to look like part of the crowd.
There are only so many ways to carry out atrocities like the Boston bombing and of course many different types of group could converge on the same modus operandi. But this particular MO looks like Hezbollah.
UPDATE: As of Friday morning, the speculation in this post appears to be wrong.
Heckuva job Russian War on Terror.