If you're a criminal prosecutor would you want the perp to know what you had on them?

[Effete Snob]

4. At what stage of the investigation?

In order to get what "you had on them", then investigators would have had to have obtained evidence from a variety of sources, not all of which are or can be bound to confidentiality.

Here's the interesting thing about being a criminal. If you have committed a crime, you generally know what crime you have committed and where any surviving relevant evidence is located. You also know who the relevant witnesses to that crime may be.

Let's say last week you shot Two Fingers Joey in the alley, while your buddy Lefty waited in the car at the end of the alley to drive you away.

If the police keep dropping by your buddy Lefty's house and Lefty says they are asking all kinds of questions about why he was parked at the alley that night, then it's not a matter of whether the prosecution is sharing anything with you at that point. You freaking KNOW what the cops are trying to figure out because you KNOW why Lefty was parked at the alley that night.

Does that help you?