It's been awhile since my last anatomy course but I'm pretty sure there are two carotid arteries plus two others (vertebral?) that supply blood to the brain. This is why it is entirely possible to live and be asymptomatic with even with one carotid fully blocked. From this I'll speculate that the knee to the neck likely has little to do with the death.
That is not to say the officer(s) have no culpability in the death, just that the most unsettling images are not the cause in my opinion. They make for a good case of abuse but are not a contributor in the death. Instead, I feel it is more likely the cause of death will be excitable delerium or positional asphyxiation. Both are not common but officers should be aware of the possibilities.
Excitable delerium is often the result of cocaine, meth, and some designer drug usage. It often presents with violent/aggressive behavior, panic/paranoia, and other behavior issues. The dangerous aspect is elevated body temperature which can become even worse upon being restrained. Recognizing the symptoms early is very difficult and once the process starts, mitigation becomes nearly impossible and the end result is commonly death. Needless to say, arm, humid weather is not beneficial to preventing this from happening.
Positional asphyxiation is maybe less rare but still not common. It is basically suffocating like having a boa constrictor holding you. Every time you exhale, the chest tightens up and one draws in less air. One can talk and yell but it becomes harder and harder as time goes on. It seems to occur most often when a suspect is cuffed behind their back and then placed face down. There appears to be no rhyme nor reason to why this happens, all body types and shapes seem to be affected.
I suspect one of these two examples will be found to be the culprit. It makes the death no less tragic nor the officers less culpable but it is likely not what it appears to be.
