It seems that the cause of Fredo's death was common knowledge among the Corleones. It is curious that Mary would ask Vincent if it was true that Michael killed Fredo. After all she probably heard it form her mother and her brother. Connie certainly knew, and Hagen and Neri knew from the beginning.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
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