Today, a letter writer who once interviewed the spy claims that to the dead man's knowledge, U.S. authorities never approved Diem's assassination.
The letter writer insisted that the US approved Diem's assassination. 
mnli hypothesis: The letter writer insisted that the US approved Diem's assassination.  premise: Today, a letter writer who once interviewed the spy claims that to the dead man's knowledge, U.S. authorities never approved Diem's assassination.