It was Sept. 18, 1961. I was 16.

Over the next hours, my mother and sisters and I learned that Mr. Hammarskjöld, accompanied by Dad and 14 others, had flown from Leopoldville, in the Congo, to Northern Rhodesia (now Zambia); that the plane, a DC-6, had not landed at Ndola, its destination; that an unexplained 15 hours went by after the airliner passed over the Ndola airport and before its wreckage was found lying not far from the runway; that all on board save one were dead.

My father, Heinrich A. Wieschhoff, was one of Mr. Hammarskjöld’s political advisers. Their party was headed for talks with the head of the breakaway Congo province of Katanga in hopes of quieting the fighting that had broken out between UN peacekeeping troops and the largely mercenary-led forces backing Katanga’s secession. It was a dramatic moment in the history of this mineral-rich country — a year after it gained independence from Belgium and quickly became embroiled in a violent quagmire involving the interests of not only Belgium but also France, South Africa, the Soviet Union, Britain and the United States.

Days after the crash, we learned that the sole survivor had died. Now there was no one to shed light on what had occurred. My family’s experience was lived in one wrenching way or another by the families of the 15 other victims. The particulars were different; the pain was the same — and only worsened because no one could tell us why the plane had gone down.

From the outset, there were legitimate concerns about the possibility of foul play. Within months of the crash, three inquests were held in rapid succession. The report of a UN commission, relying to a large degree on groundwork done by the-then Rhodesian Federation, was inconclusive, as was a report by the federal civil aviation body. The report of a commission empaneled by the Federation arrived, by a curious turn of logic, at the convenient conclusion that the event was an accident.

At first we assumed the UN would be vigilant in looking for new clues and dogged in running them to ground, and for years that seemed to be the case. Dad’s UN associates fielded our questions about the results of the original investigations and new allegations of wrongdoing promptly and graciously.

Once those associates left the UN, however, I gradually began having doubts that anyone in a leadership position cared much, if at all. One exception was Jan Eliasson, the deputy secretary-general under Ban Ki-moon, who was seemingly alone in advocating a serious look at the death of his idol and fellow Swede, Mr. Hammarskjöld.

The UN’s public posture toward Mr. Hammarskjöld drips with veneration — naturally. Yet when it comes to actually unraveling the circumstances of his death, a certain callousness prevails, despite high-sounding pronouncements to the contrary. In my experience, concern about the other 15 victims is even lower.

One byproduct of this indifference has been a coming together of nearly all the families of the deceased. Partly as a result, I have sensed that the UN is paying more attention to their interests, at least in its public comments. Privately, I still encounter telltale signs that the organization views the search for answers as a housekeeping matter.

For instance, when a group of the relatives sent the UN Secretariat a copy of a letter thanking the UN members sponsoring a recent resolution bearing on the crash, the response was a form letter from the public inquiries team stating that “the matter you raise is one of domestic jurisdiction, and does not fall within the competence of the United Nations.”

In 2011, the inquiry hit a turning point. Susan Williams, who had no prior connection to the crash, published “Who Killed Hammarskjöld? The UN, the Cold War and White Supremacy in Africa.” A sobering probe of information that the three post-crash inquests did not have, or had but failed to consider properly, it presented the UN with a chance to dig deep.

Dr. Williams, a historian and senior research fellow at the University of London, did not identify a likely cause of the disaster, but she did present a number of startling claims, including that US intelligence services allegedly eavesdropped as an unidentified plane attacked Mr. Hammarskjöld’s during its landing approach.

The book sparked hope that the UN would finally give the crash its due. First, however, a group of private citizens established a pro bono commission of four jurists to evaluate her findings. In 2013, they determined that significant new evidence could justify reopening the UN’s original investigation.

The stage was set, at long last, to bring this unhappy affair to a definitive close. Unfortunately, instead of insisting that further exploration be unlinked from the agendas of individual member states, and Secretary-General Ban be given a free hand to deal with the crash as he saw fit, the office of the secretary-general solicited the views of certain members of the Security Council. Predictably, influential members signaled their lack of enthusiasm for a full-fledged re-opening of the investigation.

In other words, the UN ducked — in my view, avoiding discomfiting questions about the roles of Belgium, France, South Africa, the Soviet Union, Britain and the US in events related to the crash, and possibly about the UN’s own handling of its original investigation and subsequent new evidence as well.

What followed was five years (and counting) of a piecemeal, woefully ineffective process fashioned to give the impression of rigor. Through resolutions organized by Sweden, the General Assembly first relegated the crash to a “panel of experts” for yet another assessment of new information (2014), then to an “eminent person,” the former chief justice of Tanzania, Mohamed Chande Othman, for follow-up (2016).

The resolutions asked member states to search their archives for relevant material and to declassify sensitive records, namely intelligence and military files. But genuine cooperation from the key players has been slow and halting. Russia and the US, as of a recent date, failed to comply fully with the General Assembly’s resolutions, and South Africa and Britain appeared bent on frustrating the process altogether. To my knowledge, the UN has rarely generated information on its own, so that leaves Chief Justice Othman to rely heavily on private sources.

As far as I am aware, the Secretariat has not engaged at a high level with recalcitrant member states to get them to adhere to the General Assembly resolutions. It has done little to publicize the activities of the chief justice. It has been slow to fully declassify its own archives and still refuses to release some documents.

In their Dag Hammarskjöld Lectures, in Uppsala, Sweden (Mr. Hammarskjöld’s home base), Secretaries-General Ban and António Guterres each mentioned the search for the truth about the crash but at the tail end of their presentations, almost as an afterthought. Instead of taking a meaningful stand, they repeated the hollow refrain: the UN was doing all it could do to find answers and member states should comply with the call to declassify relevant records.

Equally revealingly is the fact that in 2017, Secretary-General Guterres’s office sought to end the Judge Othman probe. Thanks to Sweden’s insistence, the General Assembly renewed his appointment. Did the secretary-general tip his hand last year when, rather than appear in person before the General Assembly, he sent a subordinate to present Judge Othman’s interim report?

His findings were impressive, especially considering his meager support. For his current engagement of about 15 months, Judge Othman has only himself and an assistant, working part time and in different countries, on a budget so small that nearly a third will go toward translating his reports into the UN’s official languages.