A three-member panel has presented its final report on the Swedish diplomat's death to Secretary-General Ban Ki-moon, who plans to make it public soon.
The investigators traveled to Zambia to meet with "new witnesses" and "successfully gathered additional new information" from governments and from private archives in Belgium, Britain and Sweden, said spokesman Stephane Dujarric.
Hammarskjold died at age 56 when his plane crashed on September 17 or 18 1961 near Ndola, in northern Rhodesia, now known as Zambia, as he was on his way to negotiate a ceasefire for Katanga province in Congo.
In September 2013, a commission had called on the United Nations to reopen the investigation, saying there was "convincing evidence" that the UN chief's plane was shot down as it prepared to land in Ndola.
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Witnesses questioned by the commission spoke of the presence of another aircraft that fired on the DC-6.
The commission, which was composed of jurists and diplomats, also asked the US National Security Agency to provide access to recordings it may have of conversations in the plane's cockpit and radio messages that the crew may have made in 1961.
The UN General Assembly in late December adopted a resolution, drafted by Sweden, that called for the new investigation to finally shed light on the top diplomat's death.
The panel's findings will be released once privacy considerations are addressed and Ban is to outline options on the way forward, said the spokesman.
Led by Tanzanian prosecutor Mohamed Chande Othman, the panel also included Kerryn Macaulay of Australia and Henrik Larsen of Denmark.