The police may have been negligent in not securing him back there. Then they did not get medical help when I heard he asked for it. That is not manslaughter but it needs to be corrected and it does leave them (the city) potentially financially liable, if true.
The trouble with securing is that belts can be used to commit suicide. You just wrap one around your neck and slump off the seat.
When you are transporting an out of control prisoner it is not a good idea to belt them in, it places your head where the prisoner can bite, head butt, or knee you.