Security was standing directly behind her and seemingly do little prior to the shooting.
"Security"? Their SWAT uniforms said "POLICE", and each man had a rifle and handgun. And not one of them lifted a finger to stop the scene from unfolding. Maybe their job was to keep anyone from stopping it. Also see how the MOMENT she hit the ground, a man in suit and mask with a wrist radio yelled "shots fired" and appeared right at her side.
Never thought "The Parallax View" would come to mind in a modern context, but it sure does here.