What I read was that the rounds were fired from no less than 2 feet which leaves everything out to infinity. No powder burns or residue suggest distance.
Again, consider how momentum shows itself in the pictures of the crime scene.
Brown seems to have been moving forward (ending in a face-down position). And was a really big guy. Hence a good amount of momentum. Had he been close enough to the cop to rec powder burns, he would have still ended up face down, but ON TOP of the cop.
Which is something that the investigators will need to consider. Are cops trained to wait to the last moment to shoot (resulting in their attackers, altho wounded, still plowing into them and knocking them down) or earlier (to drop the attacker before a point where momentum will carry them into you.)?