Conversation so far:
What was the accusation?
looked extensively into the problem of unauthorized and uncontrolled powers exercised by non-elected officials, specifically White.
Was the accusations found to be valid?
Attorney General Herbert Brownell Jr.
revealed that the FBI had warned the Truman administration about White before the President appointed him to the IMF.

Question: Why did they warn them?
Self-contained rewrite: Why did they warn Truman administration about White?