Accident means it was NOT done intentionally. Gross negligence is still an accident. I get the point, so that the gross negligence element is not lost as a lesson we emphasize the negligence and not simply call it an accident. And most states laws do have something like manslaughter with the grounds that the negligence was so out of reason, that the excuse it was an accident, does not relieve you for the responsibility of the gross negligence. In this case, unless the shooter is lying I expect it will be declared an accident and no charges filed, his continued employment as a Police Officer may be in jeopardy, and that is very reasonable IMO.
I saw this discussed on another forum, and the same thing appeared, why was someone else even in the room with him.
Lets no forget, unless you're going to isolate yourself a 500 yards for a pistol and 2000 yards for a rifle in all directions from everyone else, isolating yourself does not assure others safety while cleaning a firearm. True, probabilities go down with distance. I'm not aware of any safety rule saying you have to clean weapons alone. And in the military we always cleaned weapons in a group in very close proximity to each other. In recreation, I have cleaned weapons after the range in a group with the friends/family I shot with as well, I have yet to have a someone hurt during cleaning.
As more details are released about this, on the other forum, people were jumping to conclusion on all these details don't add up and were proof things were very fishy about this. I pointed out, if these two both wanted to be cops, and one actually got a cop job in a department 35 miles away, the other was still working as a security guard living with his parents and they both had a day off and got together to go shooting with each, then went to clean their weapons at the younger friends house when the accident happened. Which makes all the details coming out to fit perfectly, and is a very reasonable suspicion. No I don't know this is true, but its a possibility that should be considered before declaring a crime went down because the facts don't fit your imagined scenario.