something is not right here - bad journalism ?
he died of "asphyxiation
asphyxiation occurs as a result of a lack of oxygen, usually due to a blockage of the airway by some means.
this happens neck and below.
compression to the neck and back, caused by officers kneeling on him, led to a lack of blood flow to the brain.
lack of bloodflow to the brain is not asphyxiation. the lack of bloodflow to the brain can cause stroke, seizure and brain death - this is not asphyxiation.
i suspect death arose out of brain injury.
brain injuries set off a sequence of events, the brain seeking to save itself above every other organ.
a lack of adequate oxygen being delivered to organs causes heart rate to ramp up in an attempt to deliver oxygen rich blood to the organs, as demand continues unsatisfied, so does heart rate increase - while this is all happening, the heart itself is being deprived of oxygen. cardiac arrest takes place, followed shortly thereafter by brain death.
toxicology results should provide more clues which really become academic.
either the airway was obstructed, or carotid arteries - or both, as a result of external pressure.