Yeah, it's got to be something really big, or a lot of incriminating things. Maybe all of the above.
They sure as hell wouldn't take KAK's word for anything, and this was a guy who looked really, really good for this:
KAK is a sexual deviant, had communicated with them, may have planned to meet them, and he appears to have left for Vegas quickly after.
So instead, we have a guy plucked right out of the blue. If it wasn't KAK, with all those circumstantial pieces, then just imagine how strong the case must be against Allen.
I think it just has to be DNA, and perhaps some sort of digital trail as well.