Good question Puggle. Personally, I think the police would have found some evidence in the houses that they searched (specifically, the sister's). I remember them saying the searches were "very productive" or words to that effect. I also suspect they have credible witnesses who came forward. I remember also the police saying they had located people that they believed knew what had happened to Karlie and Khandalyce - once again, I don't remember exactly, but it was something along those lines - which made me think they found out a lot from someone who spilled the beans. I expect Daniel Holdom didn't have an alibi for the timeframe as well, plus the police information about him having previously been found to have Karlie's key card in his possession. They've probably got quite a list I would think. How much is hard evidence and how much is circumstantial is a matter only they know. But I feel quite confident they wouldn't have arrested him unless they had very good cause and are quite confident themselves. I note also that Daniel Holdom didn't co-operate, which to me is telling. If he was innocent, I would have thought he would want to make his case quite strongly, as well as apply for bail. But that's just my own personal take.