I truly believe that this problem has always been present. It is just that, with the advent of cell phones and the web, we are seeing more documentation. You can't alibi away much of this. Nothing justifies kicking a prone suspect in the head.Define "tangible". Is it something written or can it be an "unwritten rule" or predominant attitude - even if on a local level? Can it be the well documented intentions of a law? Can it be selective enforcement? Can it be creating laws with a certain intent, selectively enforcing those laws, then pointing to the need to more intensely selectively enforce in high crime neighborhoods?
Truth be told, I think we have a police brutality problem in this country, period; but there is an intersection with racism.
I do agree with looking at the circumstance as a whole, and what precipitated the escalation of violence, but many times the adrenaline and anger kicks in, and, voila, you have another incident on your hands.