The FBI held the Unibomber note secret for a long time. Once they released it, his brother recognized him in it.
But cops, like all bureaucrats, protect their turf, so they'll not release the info until the press forces them into it, as here happened with the threat to our children.
Release the note and the audio.
The problem is that the murderer asked him not to make it public. And if he gets displeased with Ch. Moose, then he just kills another person. I believe the the chief weighed the need for the public to know about the threat to children against the possibility of another shooting. Difficult decision to have to make!