RJ himself has said that ALL of the neccesary clues can be found in The Fires of Heaven. This being the case, there is NO POSSIBLE WAY that the culprit could be: Osan'gar, Aran'gar, Shaidar Haran, OR Moridin! None of them appeared before Lord of Chaos. (Anyways, aren't those four just too common for answers to mysteries these days?) Unfortunately, I do not currently own a copy of TFoH, and do not have a good suspect.