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.

For the people who didn't read the article, or found it confusing, here's some context, put into a timeline from the slightly confusing article:

Arturo Gamboa was marching with the crowd in Salt Lake City, when he separated from the march, moved behind a wall and returned with a rifle. Two of the march's peacekeepers saw this and confronted him with their handguns drawn. At this point, Gamboa raised his rifle into a firing position and ran toward the crowd. One of the march's peacekeepers fired three rounds. He hit Gamboa, and also hit innocent bystander Arthur Folasa Ah Loo. Ah Loo has died.

Not from the article: there's a video circulating of a bunch of people sitting in an entryway. One of the people there picks up a backpack belonging to someone else and moves away, saying "He's got a rifle in here". They call over the police, who force the guy down and arrest him.

The video is from when the crowd is sheltering after the shots were fired. Gamboa, who was injured, tried to hide inside the crowd. One of the people there noticed the shape of the hidden rifle, and the blood on his hand, took the backpack away and they called the cops over.

Edit: I went to the SLC sub on The Other Place:

Video of the first shots being fired (no gore) Note that it looks like the news article may have the confrontation / running away / firing sequence out of order. I'm gonna turn off notifications and you guys can debate this among yourselves.

Video of the guy finding the gun and calling the cops over

Video of people helping others over a wall to escape the area. Among the people helping others up: Batman!

Fuck I'd hate the be the guy who stopped him. Non-zero chance that he didn't actually stop a mass shooting, and ending up being the only party present who actually killed an innocent person. Seriously, I hope that guy has strong friend / family support and really good therapy, he's gonna need it all.

Wow sometimes there's a story that just proves that people don't bother to read the article at all. It's the third post I've seen about this story and in every single one of them at least half the comments didn't bother to read the article and have no idea what they're talking about.

"... by a member of the event's peacekeeping team". Peacekeeping. Sure.

Wow this fuck up!

What a shithole country.

Shot by who??