There's a certain amount of cognitive dissonance in suggesting that wrongdoing can be investigated without investigating people for possible wrongdoing; the whole point of an investigation (eliding pedantry about courts not doing it) is to determine whether an illegal act took place and who was responsible.
As long as they're respecting the Bill of Rights.
4th: Probable cause.
5th: Can't be compelled to testify against oneself.
6th: Informed of the accusation.
Suppose the police were to accuse you and a group of friends of some minor crimes based on circumstantial evidence. They ask you to testify your account of what you and your friends did that day. You plead the 5th, but they offer you immunity for the petty crimes, so you must testify. If you don't, they threaten you with contempt, leaving you imprisoned indefinitely until you do.
Suppose you misremember some minor details, or they believe someone else's story over yours. They charge you with several counts of perjury, with a penalty of 5 years jail time each. You can get a lawyer and fight it in court, but that might bankrupt you even if you win. They offer a guilty plea for a shorter sentence.
This is constitutionally suspect. They've basically entrapped you into perjury, regardless of not being found guilty of the initial crimes. A dedicated, well-paid prosecution can find many more obscure crimes to charge you with when they've got you under investigation. I hope you saved all your receipts, because here come the tax audits! From there, it's death by a thousand paper cuts, and you're spending over a decade behind bars.
For these reasons, we should really be wary of how far we allow the justice system to be twisted.