JPat Brown, B. C. D. Lipton, and Michael Morisy at Literary Hub:
The Bureau had received credible evidence regarding Sontag’s trip to Hanoi—a feature in Esquire describing her experiences talking to leading figures of the North Vietnamese government, including Prime Minister Pham Van Dong, that would later be adapted into the book Trip to Hanoi.
Though the New York office did produce a comprehensive report on the origins of the article and all of Sontag’s previous antiwar activity (citing a total of nine informants, whose names remain redacted) within the two week deadline, the investigation into Sontag stretched on for another two years. A 1971 memo from the Department of State confirmed that Sontag hadn’t sought or received permission for her travel to North Vietnam, and another memo from what appears to be French intelligence was passed on from the CIA.
more here.