[wcbtinman]

"And this guy has a clear mind, and probably some formal training, he knows how to slip away."

Then don't you think that he would also be smart enough not to leave tell-tale brass at the scene? Twice?

[Bringbackthedraft]

It wouldn't take much to pick up a recently fired shell casing at a range and leave it on the ground to mislead the investigators. Automatics eject shells, and if the shooter was half smart, he would have the sense to place a plastic bag over the ejection port to catch his spent brass. Sounds like he/she may have just dropped a casing there deliberately? I suspect that this weapon is also scoped.