Are they trained?

Yes, cops are trained to handle firearms.  And some of them keep at the training and are really quite good shooters.  Better than I will ever be.   

But the run of the mill cops aren't trained any more than you, dear reader, or our associates that might shoot a box a year.  Some cops make safety mistakes.

Like this one, locally.

Summed up:  Cop party on the 15th.  Sounds like an after work holiday get together.  11 officers there.  A gun show-and-tell.  Maybe some individuals present were unfamiliar with the model?  I could see that.  Maybe alcohol was served at this private cop party?  I can't imagine that, cops are notorious teetotallers on AND off duty.  Right?  Back to the facts.  Negligent discharge.  Sounds like the cop handling it punched a hole in his hand and the bullet went on to struck another's leg.  Happens.  Even to cops.  Maybe expecially with cops as they have that Blue Attitude.  You know what I mean.  Many of them are on the Dunning Kreuger spectrum with firearms and I've seen many LEO's act a bit more cavalierly around guns compared to other gunnies. Remember that cop on video that shot a hole through his hand and it turns out he'd done the same thing 20 years earlier?  I can't find that video but I did find another.

All 11 went on suspension since the incident.  Now 8 of those are back on duty.

(I am picturing myself as a cop at this party.  I bet I'd be in the John taking care of personal business dropping off kids at the pool, so to speak.  Then:  LOUD NOISE.  Obviously I am not involved, I was in the can.  But now I get a paid vacation for a whole week!)