The murky details of a plane crash that killed then-U.N. Secretary-General Dag Hammarskjöld five decades ago are becoming clearer thanks to a new inquiry—and investigators believe U.S. intelligence may be able to solve the case.

Hammarskjöld was en route to southern Africa in September 1961 when his plane crashed, killing all 16 people on board.

Now, an inquiry has suggested that Hammarskjöld’s death may have been the result of foul play—and that intelligence from the United States and others could hold the key to identifying those responsible.

A U.N. panel commissioned to look into the incident in 2015 submitted its report to current U.N. Secretary-General António Guterres in August. The main author of the report, former Tanzanian chief justice Mohamed Chande Othman, wrote in its executive summary that it had uncovered “a large amount of valuable new information” that pointed to the possibility that the aircraft was attacked, according to The Guardian, which has seen the report.

In particular, Othman found that the United States had deployed electronic surveillance aircraft “in and around” the site of the crash in Ndola, Northern Rhodesia (now Zambia). American spies and defense officials were also present in the region on the night of the crash, the report said.

The report also found that British and Rhodesian authorities were intercepting U.N. communications when the crash took place, meaning that the U.K. may have relevant information.

Othman’s report found that testimonies of local witnesses—who claimed to have seen another plane and flashes in the sky—was disregarded by previous inquiries. Othman also found evidence that supported an account given by a French diplomat, who said he had been told in 1967 that a Belgian pilot, working as a mercenary for the rebels, had fired at Hammarskjöld’s plane to try and stop it from landing but mistakenly clipped its wing.

“There is a significant amount of evidence from eyewitnesses that they observed more than one aircraft in the air, that the other aircraft may have been a jet, that [Hammarskjöld’s plane] was on fire before it crashed, and/or that [Hammarskjöld’s plane] was fired upon or otherwise actively engaged by another aircraft. In its totality, this evidence is not easily dismissed,” said the report.

Othman said that an “independent and high-ranking official” should be appointed to comb intelligence archives in Washington, London and other relevant states to search for any information that could help conclude the mysterious case.

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Hammarskjöld was flying to Northern Rhodesia in a bid to stabilize neighboring Democratic Republic of Congo, which had become independent in 1960 but was convulsed by a secessionist rebellion in the mineral-rich Katanga region.

The U.N. leader’s plane was flying from Kinshasa, the Congolese capital, to Ndola, where British officials were hosting talks with rebels from Katanga. The plane crashed on its approach to land, and everyone on board died.

Previous inquiries run by British authorities blamed pilot error for the crash, while a 1962 U.N. probe reached an open verdict. But independent researchers have long alleged that foul play may have been involved in the crash.

U.S. President Harry Truman, who was president at the time of the crash, told reporters the day after the incident that Hammarskjöld was at “the point of getting something done when they killed him. Notice that I said ‘when they killed him’.” Truman did not elaborate on who he meant by “they.”

Guterres has not publicly commented on the report’s findings, but in May called upon U.N. member states to disclose or declassify information regarding Hammarskjöld’s death.