Dude should be charged with Negligent Homicide. But he's a cop.
Likely, alcohol was involved.
Alternatively, his story is bullshit and it is murder. Which is, IMO, much more likely.
Either way, he should be charged. Guns never "just go off".
Being Detroit, it was likely a GLOCK, but still....
If it were anyone but a policeofficerman, charges would have already been laid, and the person would be in jail for some sort of homicide.