“Looks to me like the terrorist is using a shoulder-mounted mic, possibly communicating to the other shooters. Did you notice that prior to him starting shooting?”
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Downloaded the vid and zoomed in. Could be communication, but if you’re thinking his arm is up to talk into a shoulder-mounted mic, I believe what you’re seeing is the weapon’s shoulder strap.
I am aware of the shoulder strap grab, but either he hesitated at first as to when to commence his shooting or he was squeezing his mic button. More likely grabbing his shoulder strap to swing his weapon around, but he did hesitate at first.