Most of the videos offering proof really don't, more like conjecture.
But this video which has been up since soon after the shooting shows a second gunman, watch at 50 to 53 seconds at lower left. There are several more longer videos with boring analysis but this one was there on Monday and clearly shows a guy at the left of the screen crouch and fire into the crowd, then he runs past the camera in a police/guard uniform.
I am hardly a gun nut, never owned or fired one. A ban won't affect me. They are trying the same crap with our cars if you observant enough to see.