As I can interpret, from this horribly written article:
A man (Arturo Gamboa, 24) was seen ducking behind a wall and grabbing a rifle (an AR-15 is how it's named in the story). 2 men in bright vests (peacekeepers. as in..who? It's not clear who they are) confronted Arturo with pistols drawn. Arturo raised his rifle and moved toward the crowd, and the peacekeepers opened fire. They injured Arturo and killed Arthur Folasa Ah Loo, 39. Arthur was an innocent bystander.
Arturo has been charged with murder for creating the situation that lead to Arthur's death. The peacekeepers remain unidentified.