Shortly after midnight, on September 18, 1961, UN Secretary-General Dag Hammarskjöld died in a plane crash just outside the Congo. Hammarskjöld was on his way to broker a peace settlement for the Congo Crisis, a conflict that pitted the UN against the proponents of white minority rule in central and southern Africa. It could have been what got him killed. 

SOURCES:

Who Killed Hammarskjöld? The UN, the Cold War and White Supremacy in Africa by Susan Williams

https://www.goodreads.com/book/show/12657762-who-killed-hammarskj-ld-the-un-the-cold-war-and-white-supremacy-in-afr

Cold Case Hammarskjöld by Mads Brügger and Göran Björkdahl

https://www.imdb.com/title/tt9352780/

The Assassination of Lumumba by Ludo de Witte

https://www.versobooks.com/books/792-the-assassination-of-lumumba

U.N. Report Bolsters Theory That Hammarskjold Plane Was Downed 

https://www.nytimes.com/2019/10/08/world/africa/dag-hammarskjold.html