It could also be a case of a prosecutor who agrees with the shooter. (A right-wing extremist prosecutor, who has ever heard of such a thing?)

In that case, the prosecutor might feel pressured to bring the case before a grand jury, just to make it look like he's doing his job. But he could deliberately throw the case, neglect to mention important evidence, etc, etc, and fail to get an indictment. That way, he gets to shut down the prosecution without making it look like it was his choice. Since grand jury proceedings are sealed, nobody would be able to know he deliberately sandbagged and failed on purpose. Then he gets to make a public statement about how he tried, but the grand jury said no, so his hands are tied.

So it could be a way for a malicious prosecutor to kill/bury the case without looking like he's deliberately letting a murderer go free.