In The Golden Thread: The Cold War and the Mysterious Death of Dag Hammarskjöld (Twelve, July.), journalist Somaiya probes the diplomat’s death in a 1961 plane crash in Africa.

What drove your interest in this case?

Hammarskjöld. He was chosen as the UN secretary-general because he seemed to present no danger—he was a brilliant economist and a deft bureaucrat, but was seen as weak and easily influenced. What the world’s superpowers failed to bank on was that he was also eccentric, idealistic, and creative. When the Congolese rioted for democracy in 1959, it was really not a popular or easy thing to stand up for their right to form their own independent government, as Hammarskjöld did. It meant defying Soviet Russia, significant elements in America and Britain who wanted to run the country to their advantage, a band of international white supremacist mercenaries who felt the Congo should be run by white Europeans, and the mining companies which funded those mercenaries in order to keep extracting the Congo’s minerals.

Why did he have so many enemies?

I think there were a lot of people at that moment who felt they had to maintain an illusion or a belief—communism, capitalism, the superiority of one race over another—at all costs. They were zealots. And there’s nothing zealots hate more than someone like Hammarskjöld, who can see clearly, and who’s willing to act in the service of something higher than tribalism.

What new discoveries did you make?

The keys to this mystery, in my opinion, lie in unseen evidence from the now-defunct Federation of Rhodesia and Nyasaland. It was run in collaboration with Britain, and was something of a stronghold for white Africans who felt that they alone understood how to manage the continent for the greater good, and also a hub for British and American spying on the Congo in 1961. Until recently, relevant papers from that federation were considered lost forever, burned, or moldering unknown in some basement. But, via the Zimbabwean government, I found some eye-opening pieces of evidence that detail shocking hostility and suspicion toward Hammarskjöld and the UN, and hint at plans to act on it.

Do you think the truth is attainable?

I certainly think we can hope for an honest, accurate, and fair account that reveals everything we can possibly know today, including information we know to exist in the possession of American, British, and other national governments. The UN itself has persuaded them to loosen their reflexive grip on their Cold War secrets, and it shows every sign it will continue to push. As will I.