There is overwhelming evidence that the US and Britain supported Katanga's secession. The objective for which the UN was invited into the Congo was to restore law and order and expel foreign troops in Katanga (including 250 Belgian "military advisers" and over 300 other white mercenaries). The UN was, in effect, in the Congo to end the secession of Katanga.
But this secession had been made possible by the financial support provided to the Katangese leader, Moise Tshombe, by a Belgian mining company closely linked to British and American financial interests. Tshombe was prodigiously promoted as a Western "good boy", in opposition to the "Communist stooges" who had succeeded the Congo's first prime minister, Patrice Lumumba, in Leopoldville, under Antoine Gizenga.
So Western Cold Warriors put very strong pressure on Hammarskjold to undermine UN efforts against Tshombe's forces in Katanga. Hammarskjold agreed to fly to Ndola, in Northern Rhodesia (now Zambia) to see Tshombe. On his way there, his aircraft crashed and he was killed.
In the light of the perception in the highest Western circles that the UN was carrying out its mandate in the Congo to the detriment of Western capitalist interests, many Africans believed at the time of Hammarskjold's death that it would not have taken much for Western governments to at least close their eyes to any plans made towards dispatching Hammarskjold, by the same shadowy intelligence agencies that had murdered Lumumba.
It is to be hoped that President Nelson Mandela's government will leave no stone unturned to unravel the mystery surrounding Hammarskjold's death. For it was a signal event in the history of modern great power politics.