IRS agents have access to information that I was able to use to turn otherwise uncooperative witnesses into fountains of useful information. It is the kind of information that requires one to declare income streams, sources and amounts under penalty of incarceration. It is the kind of power that gets abused easily. Hardly unessential to the criminal investigator. Mostly unnecessary since there are other ways of structuring a tax than to club society over the head with constant threat.