Still, I won’t be surprised to find the FBI etc to be looking for someone with a sniper background.

A trained sniper would be ashamed of that shot. Latest info I have is the shot was from 430 feet. About 140 yards. I was "plinking" soda cans at a shooting range at 100 yards with iron sights (I would hit the cans about every other shot, so 50%) with my dad. At 140 yards, a human sized target wouldn't be that difficult if one is braced. However, I suspect the shooter had a scope and was going for a head-shot. He missed. But it was still effective.

If I were to guess, I'd say it was someone with some hunting experience. The rifle found was a "bolt-action", which would be the choice of a serious hunter. I didn't find any mention -yet- of the caliber, and whether it had a scope. A military sniper would have taken his head off.