Just speculation here, but maybe this..
Assume LE does not have a body, but they do have other direct evidence that was used to obtain the indictment. I'll call it "ZA's trophy".
We know ZA hasn't confessed, so assume his defense is something like "oh yeah I found this "trophy" in the roadside ditch by my house at the time of the initial searches, and I kept it, which makes me a dirtbag, but not a killer". I mean something like that is going to be a tougher nut to crack for the prosecution, but its the obvious lie on ZA's part.
LE needs to try a different approach to locate Holly, and knows or suspects that others knew about the trophy, assume ZA is hopped up and bragging or something to that affect, and maybe he tells a little more than he should about the location of the body at the same time, to some drug head, or person who themselves wouldn't do such a crime, but for other reasons is adverse to LE, due to drugs or whatever. Now LE puts it out there that, hey, the game is up, come tell us what you know, or otherwise you will always be waiting for the other shoe to drop. This person/people care more about themselves than about ZA, and maybe want to stay free and high and didn't want anything to do with this in the first place, so they come it and spill. On the other hand, if LE goes and gets them, and tries to put the pressure on, they can always say "oh yeah well we just thought ZA was on drugs and he never said anything about where the body was.."
Maybe, Maybe not, the point I can see to it is that its using the guilty conscience as a weapon, right, and if they come forward on there own then a free will statement maybe they get off, and LE has a more credible lead. On the other hand if LE backs them into a corner and they start down the lying path everything gets harder...
A free will statement is less likely to be a lie, IMO. Again all this is just speculation. I hope they have found her and this horror can finally be over in that respect anyway.