An attorney, in an American Thinker piece, discussing how to prosecute the IRS for its partisan activity made this statement:
One of my favorite tactics (I think it is okay to reveal it) was to take the deposition of the lowest significant person in the office, not the highest. An official's assistant is usually unwilling to lie under oath and risk perjury for his or her political boss. Top officials are often skilled liars. Their assistants will usually sing like canaries.

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Seems just as applicable to the other investigations that should be taking place (though it appears that none of those investigations are not being very rigorous).

He lays out an interesting approach to compel the IRS to reveal its documents pertaining to the allegations.