Grand juries are notoriously biased towards the prosecution, if they want to bring charges or not. In this case they clearly wanted to just ignore it; nothing happened until this video emerged and the murder itself happened a couple of months ago. I suspect that the old saying about grand juries being able to indict a ham sandwich will apply here, but in reverse.
This does seem to be turning into a massive media storm though, so this might force them to actually bring charges. If it ever even gets to court there don't seem to be any witnesses other than this, as yet, anonymous cameraman and the two shooters themselves, one of whom used to be a cop, and with the "stand your ground" bullshit card already having been played, it seem there will be a replay of the Travon Martin case, at least as far as the result goes.
I actually think if they get charged they will get some generous plea deal offered to them; something like 3-6 for some kind of manslaughter and have it suspended. At least they can wash their hands and say they didn't just ignore it.
It would be an insult to banana republics to compare the U.S justice system to theirs.