The shooter was a veteran. That’s likely more than just basic experience with firearms. If you’re good all you need is simple to do lots of damage.
One of my running buds is a former Marine sniper. Trust me, he could take out a lot of guys from one strategical elevated position. The training turns you into one deadly killing' machine. He was in 8 years and was assigned to protection details on high level people when they passed through his territory.
When I saw the footage of that veteran moving around I told my wife that guy moves like ex-military and when I saw him shoot the cop like he was nothing I knew he'd done combat time. One doesn't move from cover to cover the way he did with armed police so close by. He'd done this before. I wonder if any calls had been placed by anyone in the current administration to any of the New Black Panther's members before this went down...