Here, at Constitution.org I found this:
Sec. 149. THOUGH in a constituted common-wealth, standing upon its own basis, and acting according to its own nature, that is, acting for the preservation of the community, there can be but one supreme power, which is the legislative, to which all the rest are and must be subordinate, yet the legislative being only a fiduciary power to act for certain ends, there remains still in the people a supreme power to remove or alter the legislative, when they find the legislative act contrary to the trust reposed in them: for all power given with trust for the attaining an end, being limited by that end, whenever that end is manifestly neglected, or opposed, the trust must necessarily be forfeited, and the power devolve into the hands of those that gave it, who may place it anew where they shall think best for their safety and security. And thus the community perpetually retains a supreme power of saving themselves from the attempts and designs of any body, even of their legislators, whenever they shall be so foolish, or so wicked, as to lay and carry on designs against the liberties and properties of the subject: for no man or society of men, having a power to deliver up their preservation, or consequently the means of it, to the absolute will and arbitrary dominion of another; when ever any one shall go about to bring them into such a slavish condition, they will always have a right to preserve, what they have not a power to part with; and to rid themselves of those, who invade this fundamental, sacred, and unalterable law of self-preservation, for which they entered into society. And thus the community may be said in this respect to be always the supreme power, but not as considered under any form of government, because this power of the people can never take place till the government be dissolved.
Sec. 156. The power of assembling and dismissing the legislative, placed in the executive, gives not the executive a superiority over it, but is a fiduciary trust placed in him, for the safety of the people, in a case where the uncertainty and variableness of human affairs could not bear a steady fixed rule: …that the limited time of their sitting might be too short for their work, and rob the public of that benefit which could be had only from their mature deliberation.
Many of the cases I am reading on “impartial investigation” have to do with the “mature deliberation” of the fiduciaries.
From the above quotes we can see that all government officials, even IRS agents, are fiduciaries.