**Spoilers** Warning up front, if you haven't read all of The Starter, this could contain some spoilers.
So, in the book we learn that the Zoroastrian Guild is responsible for the bombing at the parade. Since we can say that Quentin is definately not the fallen ZG member...who is? The book presents the theory that someone on that train was a ZG memer, fell away, and the bombing was revenge. So if it's not Quentin who do you think it is?
I have my own theory on who it could be if that is indeed what the motive behind the bombing was, but I am interested to see what you guys think too.