Grand jury indictments are required for felony charges to make it to trial, including felonies like murder/involuntary manslaughter.

Indictments are a very low bar (probable cause). In this case, it seems clear to me from everyone’s accounts that, at minimum, this was a reckless homicide where the mishandling of a firearm resulted in someone’s death, and therefore probable cause existed to indict, so this is very clearly a poor decision on the jury’s part if the charge was manslaughter. I’m not sure if they tried to seek an indictment for involuntary manslaughter or murder though. Murder is a higher bar.

However this isn’t necessarily a done deal. Double jeopardy does not apply to grand juries’ “no bill” (i.e. the decision not to indict), so the prosecutor can gather more evidence or plan a different approach and try again. If, for example, they attempted to get an indictment for murder and failed, they could try again for manslaughter. This is really only news if the prosecution decides to stop trying to indict.