Drunk Chicago cop mysteriously shoots another one

Shrouded in "mystery" two Chicago police officers get in an argument out at the bars, one gets shot in the head.   A lawsuit is now filed. 

The men left the bars and went to Kelly’s home, where the off-duty officer’s service weapon discharged while he was intoxicated, according to the suit.
The suit does not specify how the gun discharged, but does claim Kelly engaged in an unjustified argument and fight with LaPorta. It does not indicate when the argument and fight happened in relation to the gun discharging.