It was never confirmed to my knowledge, but the hints given in the series are compelling. Noal often tells stories of his travels, in the Borderlands and even Shara. Olver talks with him about Jain's greatness, and Noal says that Jain let himself be used by someone, but doesn't say who. That would likely be Ishamael.

It is possible that Brandon let something slip once in a blog post, or maybe on the wotmania MB. I seem to remember something like that, but I'm not certain.