I would probably say that both the city and the cop should, independently, be liable. Given the position of authority the city provides, it is ultimately responsible to hire and properly train people who will use that authority well, while the individual is also responsible for their own actions.
If the cop is following procedure, the city and others who set the procedure should be liable. If the cop is breaking procedure, then they should be liable. If there is no clear procedure, then they should both be liable.
Both is good with me, yes.