Many Americans believe the role of the police and the prosecuters is to search for the truth.
The purpose of the police and the prosecution is to build a case.
Another thing people get confused on, in theory, you are innocent until proven guilty IN A COURT OF LAW.
LEO's tend to assume you're guilty.
That may be the purpose that police and prosecutors have acquired over the years, but that is not the purpose they are supposed to have under our constitution and legal system. That distorted purpose is another example of abuse of power by government.