investigations investigate. If they discover criminal liability, documet it  

investigations investigate. If they discover criminal liability, document it and if that evidence of criminal liability rises the the level of persuading a jury, then it will be prosecuted by the relevant authority. If it is not immediately a justified case but the data indicate that there is evidence of a crime that could be proven with additional facts, then an agency could investigate.

Either way, the committee has performed a useful service to justify their existence.

Not having prosecutorial ability can enable an investigation to be a more thorough, complete investigation. Would you offer information to an investigator that could result in your prosecution or that of 'friendly' body?

The cliché is simplistic but it has some relevance- "If you are innocent then you have nothing to hide and will be forthcoming".

What does one immediately assume if somebody refuses to answer a question because it may tend to incriminate themselves? Something is being hidden.