I know where he died and at this moment in time his killer to me is unknown. What I want to know is who did it. Robert Jordon claims there is enough information there to figure it out but I'm not sure.
That's what I mean by no known killer. We're never officially told At least, I don't remember it. And Asmodean's biography in the Complete Guide to the Wheel of Time or whatnot does not list a killer either. But as for me, I haven't figured it out, nor have I seen enough evidence to point me straight to someone.
Book 5, Fires of Heaven, Last Chapter "Glowing Embers" or something like that. Book 5 is the book to study carefully... Book 5 and Book 4. Because for it to be obvious based on the evidence given you have only use the knowledge you have from the first 5 books and nothing afterwards.
Talking about Robert Jordan. In case people here didn't know it, he's been suffering from a serious disease for some time. You can follow his fight against it on his official blog: http://www.dragonmount.com/RobertJordan/