I'm a varmit hunter, rintense, have been since age 11 (back when there were many "farm kids" running around using high-powered rifles to dispatch various destructive animals who would prey on our valuable stock.) Basically, a well trained military sniper is a varmit hunter - you can quickly see the parallel.
I'm not as steady as I used to be, but woe unto a coyote who goes after a calf or sheep and is within about 400 yards, on a reasonably calm day.
This terrorist in Maryland isn't necessarily all that good, but he's disciplined and uses the basics of sniper rules, which are based on "one shot and change locations."
This guy may never be caught if he sticks to the rule of taking time to be sure he's not observed prior to the shot.