LE, the detectives, etc, are not attorneys. Whereas I'm sure over time, they gain knowledge of what will fly and what will not, it's the DA's office, with its attorneys, who determine what they can, and can't, and want to, and don't want to, take into the courtroom and present.
According to a detective friend of mine, LE reaches a point where they think they have what the DA will need/want. They have a meeting with the DA and review it. The DA says "I can use this, and this. I can't use that. I can't use that unless I also have 'x'. But what I really need before I can (charge or go to trial) is a big fat 'y'. Without that, I can't even charge. What I want, what I would like to have, is 'a' or 'b' or 'c'."
You get the idea. LE goes off and tries to find what the DA needs primarily, and what the DA wants hopefully. They have another meeting and review. Same as above.
Eventually, either the DA says, "Okay, I think I can convict on this pile of evidence. We're good to go. Get the warrants and book 'em, Dan-o." Or, if LE just can't find what the DA needs, it becomes a cold case.
One of our verified criminal attorneys could articulate all that much more eloquently that I did, I'm sure. lol.
Right now, to me, it's obvious that the DA has sent LE back to the drawing board, to those 4 binders, to pound the pavements, and find them what they need.