Two things...
First, I believe that the prosecutor could present the case again, even to a different grand jury. No double jeopardy or anything along those lines to stop it. Most times, a case that gets no billed is seen as anwaste of time. Somehow, with a dead officer, I don't think it will be seen as a waste of time. Frankly, an indictment wouldn't shock me in the least. Not that hard to get an indictment when the prosecutor controls exactly what the grand jury sees and hears.
Second, I find it interesting that so often I read posts that lambast LEOs for being "geared up oike they were on a kilitary raid." It's a job where you are going into an unknown situation where the people on the other side of the door will very likely be hostile to your presence. They may well be armed, which it appears was known in this case. Regardless of whether it is knock or no-knock, wouldn't you wear that gear if you were knowingly going into that type of situation? I know many regulars here have the plan, and the gear, to suit up in a similar way in a SHTF situation because of the possibility of engaging hostiles(got no problem with that). I wore body armor back when I just tagged along on warrants and waited outside. I carried my Glock with extra mags, and once or twice kept a shotgun real close. It wasn't a cool factor. It was being prepared.
All that said, I don't like no knock warrants either, even less the longer I sit at the bench.