What exactly needs investigating?
There is no situation where kicking anyone in the head is not criminal, and for a police officer to do so is 10x worse.
There is no context or set of circumstances that could make it in any way acceptable.
Investigation = 'Delay, hoping people will forget and calm down over this terrible conduct that is allowing other police to close ranks and try to explain the situation'.
Attacking a police officer, of course is serious and needs punishing. A police officer inflicting physical violence in a way that is always inappropriate, given the powers dynamics is as bad as it gets.
There is nothing to investigate! what are you believing the investigation could find that would justify the unjustifiable?