Conversation so far:
What was the accusation of Harry Dexter White?
Harry Dexter White was accused in 1948 of spying for the Soviet Union.
Was Whittaker Chamberse's accusations about Harry Dexter White found to be valid?
Chambers produced documents he had saved from his days as a courier for the Soviets' American spy-ring. The FBI Laboratory established that a memorandum was written in Harry Dexter White's handwriting.

Question: Why did they warn them?
Self-contained rewrite: Why did Whittaker Chambers warn officials about Harry Dexter White?