A skeleton key has to at least be one of the tools. I don't think it would be easy to get into two rooms without it unless they followed the victims around constantly. Another is obviously the poison. The only thing about that is it's possible they found it lying around the house from somewhere that was missed in the initial dumping by Beat.

Maybe a murder weapon? The only way we would be able to tell it by clarifying the way Belarus was cut and what sort of sharp object could have been used for that.