I think that it is Brett, but Frye is involved somehow. Here are a few theories:
1: Maybe, Frye and Brett used to be lovers, but now they split up, and RJ is after Jane for revenge (although this isnt likely)
2: I dont think atall that Frye is dead (sorry cotdepalbo) and getting her back will be a really big part in RJ's plan. Maybe RJ is Jane's father, as he has many reasons, although I don't see what killing Jane's wife was for. Maybe he wants Jane for himself, and that is why he has taken Frye. (And killed Jane's wife)

Also, I think that it is possible that one of the team are involved, but I really hope not!!!!

I don't really think that I like Frye-she seems strange-and maybe if she is involved, that would give her and Jane a reason to split. I hope that happens because Lisbon and Jane belong together!!!!