In 1998, documents were discovered which detailed an alleged plot, named Operation Celeste, to assassinate Hammarskjöld. The alleged plot was backed by the CIA, MI6 and a Belgian mining interest and the assassination was to be carried out by the South African Institute for Maritime Research, a South African paramilitary organisation.

One of the documents stated that CIA director Allen Dulles agreed that "Dag is becoming troublesome ... and should be removed" and pledged local CIA support for the alleged plot. The information was contained in a file which the South African National Intelligence Agency turned over to the South African Truth and Reconciliation Commission in relation to the 1993 assassination of Chris Hani, leader of the South African Communist Party. The authenticity of the documents could not be substantiated because they were copies instead of originals. More documents related to the alleged plot were discovered by the South African government in 2016.