Wasn't it obvious? It was so obvious! Anyway, the butterfly case is finally solved in this episode, almost. But I have to say, our protagonists are terrible at solving mysteries; they lack intelligence, stealth, and prudence. They stumble upon the killers almost by chance. But that's what this episode is about: they make blind guesses, terrible decisions, and people actually die because of their stupidity. I hope this is intentional, narratively speaking, because otherwise, it's really bad writing.