According to a report in the Guardian newspaper, new eye-witness testimony gathered by Swedish aid worker Göran Björkdahl supports the theory that Hammarskjöld’s plane was shot down by a second, unidentified aircraft.

Björkdahl also reviewed diplomatic communications from the weeks prior to the crash indicating that US and British authorities were upset about a UN military operation ordered by Hammarskjöld to counteract a rebellion reportedly supported by western mining companies in the mineral-rich Katanga region.

At the time of the crash, which occurred over Northern Rhodesia (now Zambia) on the night of September 17th, 1961, Hammarskjöld’s support for decolonisation had upset many of the major powers on the UN Security Council, but had earned him the backing of many third world countries.

He was thought to be assured of winning another term as UN Secretary General at the upcoming General Assembly vote, scheduled to take place just weeks after the crash.

While Björkdahl remains uncertain about who may have killed Hammarskjöld, he said he now better understands the motives behind why the headstrong Swedish diplomat may have been killed.

“It’s clear that there were a lot of circumstances pointing to possible involvement by western powers,” he told the Guardian.

“The motive was there – the threat to the west’s interests in Congo’s huge mineral deposits. And this was the time of black African liberation, and you had whites who were desperate to cling on.”

A British-led inquiry conducted at the time of the crash cited pilot error as the likely cause.

The findings, which were later adopted by a subsequent UN investigation, downplayed eye-witness testimonies from nearby villagers that there had been a second plane in the vicinity of Hammarskjöld’s DC-6 when it crashed.

But interviews conducted by Björkdahl over the last three years, with surviving witnesses from the area near the crash site, revealed that Northern Rhodesian security forces sealed off the site within hours of the crash and ordered residents to leave the area.

Dickson Mbewe, 84, described how the plane was shot down near his home in Chifubu.

“We saw a plane fly over Chifubu but did not pay any attention to it the first time,” Mbewe told the Guardian.

“When we saw it a second and third time, we thought that this plane was denied landing permission at the airport. Suddenly we saw another aircraft approach the bigger aircraft at greater speed and release fire which appeared as a bright light.”

According to Mbewe, the plane later “went down and disappeared”.

When he went to explore the crash site early the next morning, he discovered soldiers and police officers steering people away from the wreckage even though the official report of the incident claimed that the downed plane wasn’t discovered until 3pm in the afternoon.

Mikael Holmström, a security affairs correspondent with the Svenska Dagbladet (SvD) newspaper, called the new findings “sensational” despite the fact that much of the evidence cited in the Guardian report has been known for years.

“But the fact that authorities may have sealed off the site long before the plane was officially found is totally new and that indicates there could have been a cover up,” he told SvD.

Holmström also said it’s “tragic” that Sweden didn’t push harder to gain clarity regarding what actually caused the crash.

“It was clear shortly after the crash that the accident investigation was incomplete,” he said.