As you said Pedophiles have thier own chatrooms, usenet, etc etc... OK let's say that pictures of JonBenet Ramsey are being passed around, Karr sees these and is infatuated. He knows she lives in Boulder, Co from the descriptions with the pictures.
Then a light goes off. Daddies comapny or someone else he knows through those connections was doing or did work on a large home for a Ramsey family in Colorado. He puts 2&2 together.
Now he is infatuated with this beautiful little girl, and he knows exactly where she lives. To furthur speculate, maybe at this point he uses contacts, or sneaks into daddies company, etc and finds blueprints of the Ramsey home.. a plan begins to form......