In any case with a political context, or where the prisoner concerned is from a racial minority, suicides in police custiody are always open to suspicion. There've been too many cases of brutality and killing for there to be a justifiable automatic assumption in favour of the police. Sensible police officers will insist that there is a proper investigation in any such case, precisely for that reason.

From what I've seen of this case though, I'd be hard to see the motive for murder by the arresting officers. So I'd suspect a degree of negligence or incompetance rather than anything else.