Bengt Rösiö, the diplomat who investigated the case, announced in 1993 that he was certain Hammarskjöld's death was an accident and called for the investigation to be closed.
However, he took back this claim on Monday following a report in the Aftonbladet newspaper detailing previously unreleased material and eye-witness testimony which had been ignored in previous investigations.
“I am not sure of this anymore. There is so much that is unclear,” he told the newspaper.
“There are truly murky circumstances. You wonder whether the Rhodesian accident committee deliberately wanted to hide something.”
The new evidence included an image of the corpse of Hammarskjöld, which has caused some to question discrepensices between the image and the accident report.
For example, Hammarskjöld's body appears to be the only one in the crash that was not charred by the flames.
“The whole case has become truly strange lately. No one wants to know about it and no one wants to talk about it. They're keeping tight lipped,” Rösiö told the paper.
The plane crash, which occurred on the night of September 17-18, 1961, saw Hammarskjöld's DC-6 come down near Ndola airport in the British colony Northern Rhodesia (modern-day Zambia) as he flew in to peace talks to end fighting in the mineral-rich Katanga province in neighbouring Congo.
A pilot error was initially blamed in the United Nations inquiry, effectively endorsing an initial investigation by the colonial authorities, however new witness reports suggest that the plane may have been shot down.
Furthermore, Rösiö's new claims add to the widespread accusations of a cover-up and the suspicion that Hammarskjöld paid the price for supporting efforts by the newly independent Congolese government to crush an uprising in Katanga being funded by Western mining companies.
Göran Björkdahl, a Swedish aid worker who interviewed eyewitnesses to the crash in Ndola, proclaimed last year that there could be "no doubt Dag Hammarskjöld's plane was brought down."
However, Sweden's foreign ministry is not perturbed by Rösiö's new revelations.
“At the moment there are no new elements in this report,” said Anders Jörle, press spokesperson for the Foreign Ministry to The Local.
“The newly published pictures are the main development, but this was not something that was new to us. We have them in the archives, and we've chosen not to have them published for obvious and ethical reasons.”
The ministry plans to discuss the case with Rösiö before taking any further steps.
“We want to talk with Rösiö first, and listen to see what he has to add to his reflections. Then we will see what actions will be taken, if any.”
The case is not something Jörle considers to be pressing.
“It's definitely not something that's urgent. It's a catastrophe, but it's a 50-year-old catastrophe,” he told The Local.