Back in the days of Al Capone, the Bureau of Investigation (now called the FBI) investigated the actions of Al Capone. Does anyone think that the federal agents investigating Capone needed to be unbiased in order to do their jobs well? Of course not. They were lauded as good, faithful agents.
I am sure they all had a huge bias, and that was to find some way to arrest Capone. I can’t even imagine what names they called Capone.
However, their bias alone would get them no convictions. They needed facts to convict. Without the facts of tax evasion, they would not have been able to put Capone away.