The FBI. And, you already did that. I think I would stay away from that site just so you don't get involved in it.
There are organzations out there which watch over sites mostly chat rooms. I have a friend who is an agent and I can ask for names of the groups, some are connected to federal agencies and some are composed of moms. I know one was called, Childnet.
They usually will catch the guy and then, close it down. Try Childnet but I am sure the FBI will act on it, but just incase they don't it wouldn't hurt to contact them.