Why can't Susie's dad find out who murdered her in The Lovely Bones?

He does know. He
can't prove it. Susie's dad knows from the way George Harvey looks at him, and Harvey's manner
when Susie's dad helps Harvey build the tent. One of the themes of the book is how Susie is
present in spirit in her family's life--she helps her dad, Jack, to know who her killer is. Jack
can't prove that Harvey is the killer, though, much to his frustration. Lindsey, Susie's sister,
also knows, which is why she breaks into Harvey's house looking for evidence. But even the
drawing she finds does not give the police enough evidence. Jack eventually finds the strength
to let himself heal. And the way I read Harvey's death, it seemed that Susie had something to do
with the icicle falling.