>>515479457
Hey OP, i have an alternate theory (to the vent or window by the stairs).

What if ALL the people crowded on the stairs are in on it and shooter is hidden in their legs.

I keep replaying back and forth the frames around the muzzle flash. One guy obviously reacts at the same moment of the flash, and before the sound gets to the camera.

https://files.catbox.moe/vmqt4m.webm

GUY IN PIC flinches.