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..... as close as it gets to "policing themselves".
Roughly because but for public exposure, prosecutors never go after each other, no matter how bad they know the conduct. Same idea as the thin blue line, good cops never rat out bad ones. It takes some sort of external event to bring the bad ones into the light, and public demand then forces a reaction.
The internal administrative reporting is used to limit and control the damage to the institution, without exposing its inherent corruption. IOW, having the IG creates the illusion that the operation is approximately honest.
If it wasn't for some sort of public exposure, there would be ZERO consequence for the routine malfeasance and abuse of power. No agency polices itself.
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..... as close as it gets to "policing themselves".