Photo copy the diary (a dozen at least)
Talk to the Detectives and give them the diary telling them what pages to read, tell them you have copies and that if they do nothing those copies go to the press
Its not a bluff, send copies to the press a week later and a copy to the relatives a few days after that.
Oh, and stop handling the diary with your bare fingers as your prints will wipe out all others
You have evidence, you have to turn it over, if you do not and someone else dies you are responsible

