Glocks do not have a traditional saftey. I have seen this sort of accidental missfire on the range.
The "Saftey" is on the trigger:
There is no "positive action" required other than hitting the bang switch with your booger hook to make bang....
Either it was intentional or the guy is a ****ing moron; either way, it should be tried as murder.
Why the hell are BART cops carrying guns anyways? It's BART, it's San Francisco, ffs!