From New York Times, 24 September 1961, “U.N. BRACES FOR ITS CONGO TASK”:

[…]”On the fateful Wednesday morning, Sept. 13, the United Nations did little more than try to recoup the terrain it had given up voluntarily two weeks earlier. Then the explosion came.

The moment and place of the explosion, it now seems, was chosen by the handful of French, Belgian and other officers who were still in Katanga in defiance of their government’s orders. These mercenaries had made plans for effective military resistance.

One must assume they had drawn a defense line and had, in advance, made a decision, that they would strike if the United Nations troops crossed this line.

One must further assume that the United Nations troops, when they moved against the Post Office and radio station, crossed this line without realizing it and thus tripped the mechanism that touched off the explosion.

Then came Mr. Hammarskjold’s death.

Dr. Sture C. Linner, the head of the United Nations mission, lost a close friend, a man he had worshipped almost as a father. He lost his private secretary and, in Dr. Vladimir Fabri[sic] he lost what he had once called his “one-man brain trust.” Dr. [Fabry], officially the legal councilor[sic], had in fact been the mission’s thinker who analyzed events and suggested decision.”

From Tribune De Lausanne, 17 October 1961, “Mr. Hammarskjold’s Plane Was Allegedly Fired Upon”:

“Information from the Congo from a private source to the United Nations indicates that the investigators on the spot have proof that Hammarskjold’s plane was fired upon, writes L’hebdomadaire “News Week”. Several African countries, they added, intend to ask for full light to be shed on this matter.”