The racism is in the form of assumptions and predispositions. When the cops arrived on the scene, they found a dead black kid and an armed community watch volunteer, and the pieces all just seemed to fall into place. Of course, the kid had jumped the innocent citizen who had used self-defense. Now, once that was their mindset, they essentially set about defending it, tidying up loose ends, not asking too many questions, and so on.
But look, isn’t it obvious that had the dead kid been white, there would have been a lot of more questions asked? The cops would have arrived on the scene, and you would have an unarmed dead white kid, an armed hispanic dude with an arrest record. My guess is that their initial mindset would have been quite different.
Of course this is speculation but the statistics back it up but I post this more to demonstrate the subtly of racism that is often overlooked.