On Thursday, a conference in London heard an update from UN Assistant Secretary-General for Legal Affairs Stephen Mathias on the progress of the investigation, which is seeking archival documents from member states.

Participants said the United States and Britain were slow to hand over potentially vital information.

Hammarskjöld, a Swedish national, died on September 18, 1961 on his way to negotiate a ceasefire between United Nations peacekeeping forces in the Congo and separatists from the Katanga region in the same country.

Hammarskjöld's Douglas DC-6 plane crashed near Ndola in Northern Rhodesia, now Zambia, killing the UN Secretary-General and 15 other passengers.

The first investigation by the authorities in Northern Rhodesia put the blame on the pilot, but the result was controversial, as witnesses on the ground said they saw another plane flashing in the sky at the time of the accident.

According to reports, there were Belgian mercenaries in the area at the time, in addition to French and British intelligence officers. American intelligence officers were also monitoring communications from Cyprus and reported that they heard communications consistent with the United Nations plane coming under fire.

The United Nations reopened the case in 2017 under Tanzanian judge Mohamed Chandi Othman, who called for the appointment of independent officials to oversee the examination of archives in countries that may have relevant information.

The organizers of the conference, the Institute of Commonwealth Studies at the University of London, and the United Nations Association of Westminster, said: “While Belgium, Sweden and Zimbabwe demonstrated serious efforts, the responses of the United States and Britain were woefully inadequate and showed disdain for the UN investigation.”

Susan Williams, a researcher whose book Who Killed Hammarskjöld?

2011 on reopening the UN investigation: “The most recent General Assembly resolution to renew the investigation was co-sponsored by 142 of the 193 UN member states, but not by the United States and Britain.”

Former British High Commissioner to South Africa Paul Boateng said: “The work must continue because it is part of a broader struggle to uphold democracy, the rule of international law and the United Nations, all of which are under increasing threat. There must be no stone unturned to get to the truth.”

He added, "Suspicion of the murder of the Secretary-General of the United Nations is a crime that is too serious to be erased with the passage of time."