Eyewitnesses claim a second aircraft fired at the plane raising questions of British cover-up over the 1961 crash and its causes

New evidence has emerged in one of the most enduring mysteries of United Nations and African history, suggesting that the plane carrying the UN secretary general Dag Hammarskjöld was shot down over Northern Rhodesia (now Zambia) 50 years ago, and the murder was covered up by British colonial authorities.

A British-run commission of inquiry blamed the crash in 1961 on pilot error and a later UN investigation largely rubber-stamped its findings. They ignored or downplayed witness testimony of villagers near the crash site which suggested foul play. The Guardian has talked to surviving witnesses who were never questioned by the official investigations and were too scared to come forward.

The residents on the western outskirts of the town of Ndola described Hammarskjöld's DC6 being shot down by a second, smaller aircraft. They say the crash site was sealed off by Northern Rhodesian security forces the next morning, hours before the wreckage was officially declared found, and they were ordered to leave the area.

The key witnesses were located and interviewed over the past three years by Göran Björkdahl, a Swedish aid worker based in Africa, who made the investigation of the Hammarskjöld mystery a personal quest since discovering his father had a fragment of the crashed DC6.

"My father was in that part of Zambia in the 70s and asking local people about what happened, and a man there, seeing that he was interested, gave him a piece of the plane. That was what got me started," Björkdahl said. When he went to work in Africa himself, he went to the site and began to question the local people systematically on what they had seen.

The investigation led Björkdahl to previously unpublished telegrams – seen by the Guardian – from the days leading up to Hammarskjöld's death on 17 September 1961, which illustrate US and British anger at an abortive UN military operation that the secretary general ordered on behalf of the Congolese government against a rebellion backed by western mining companies and mercenaries in the mineral-rich Katanga region.

Hammarskjöld was flying to Ndola for peace talks with the Katanga leadership at a meeting that the British helped arrange. The fiercely independent Swedish diplomat had, by then, enraged almost all the major powers on the security council with his support for decolonisation, but support from developing countries meant his re-election as secretary general would have been virtually guaranteed at the general assembly vote due the following year.

Björkdahl works for the Swedish international development agency, Sida, but his investigation was carried out in his own time and his report does not represent the official views of his government. However, his report echoes the scepticism about the official verdict voiced by Swedish members of the commissions of inquiry.

Björkdahl concludes that:

• Hammarskjöld's plane was almost certainly shot down by an unidentified second plane.

• The actions of the British and Northern Rhodesian officials at the scene delayed the search for the missing plane.

• The wreckage was found and sealed off by Northern Rhodesian troops and police long before its discovery was officially announced.

• The one survivor of the crash could have been saved but was allowed to die in a poorly equipped local hospital.

• At the time of his death Hammarskjöld suspected British diplomats secretly supported the Katanga rebellion and had obstructed a bid to arrange a truce.

• Days before his death, Hammarskjöld authorised a UN offensive on Katanga – codenamed Operation Morthor – despite reservations of the UN legal adviser, to the fury of the US and Britain.

The most compelling new evidence comes from witnesses who had not previously been interviewed, mostly charcoal-makers from the forest around Ndola, now in their 70s and 80s.

Dickson Mbewe, now 84, was sitting outside his house in Chifubu compound west of Ndola with a group of friends on the night of the crash.

"We saw a plane fly over Chifubu but did not pay any attention to it the first time," he 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.

"The plane on the top turned and went in another direction. We sensed the change in sound of the bigger plane. It went down and disappeared."

At about 5am, Mbewe went to his charcoal kiln close to the crash site, where he found soldiers and policemen already dispersing people. According to the official report the wreckage was only discovered at 3pm that afternoon.

"There was a group of white soldiers carrying a body, two in front and two behind," he said. "I heard people saying there was a man who was found alive and should be taken to hospital. Nobody was allowed to stay there."

Mbewe did not forward with that information earlier because he was never asked to, he said. "The atmosphere was not peaceful, we were chased away. I was afraid to go to the police because they might put me in prison."

Another witness, Custon Chipoya, a 75-year-old charcoal maker, also claims to have seen a second plane in the sky that night. "I saw a plane turning, it had clear lights and I could hear the roaring sound of the engine," he said. "It wasn't very high. In my opinion, it was at the height that planes are when they are going to land.

"It came back a second time, which made us look and the third time, when it was turning towards the airport, I saw a smaller plane approaching behind the bigger one. The lighter aircraft, a smaller jet type of plane, was trailing behind and had a flash light. Then it released some fire on to the bigger plane below and went in the opposite direction.

"The bigger aircraft caught fire and started exploding, crashing towards us. We thought it was following us as it chopped off branches and tree trunks. We thought it was war, so we ran away."

Chipoya said he returned to the site the next morning at about 6am and found the area cordoned off by police and army officers. He didn't mention what he had seen because: "It was impossible to talk to a police officer then. We just understood that we had to go away," he said.

Safeli Mulenga, 83, also in Chifubu on the night of the crash, did not see a second plane but witnessed an explosion.

"I saw the plane circle twice," he said. "The third time fire came from somewhere above the plane, it glowed so bright. It couldn't have been the plane exploding because the fire was coming on to it," he said.

There was no announcement for people to come forward with information following the crash, and the federal government did not want people to talk about it, he said. "There were some who witnessed the crash and they were taken away and imprisoned."

John Ngongo, now 75, out in the bush with a friend to learn how to make charcoal on the night of the crash, did not see another plane but he definitely heard one, he said.

"Suddenly, we saw a plane with fire on one side coming towards us. It was on fire before it hit the trees. The plane was not alone. I heard another plane at high speed disappearing into the distance but I didn't see it," he said.

The only survivor among the 15 people on board the DC6 was Harold Julian, an American sergeant on Hammarskjöld's security detail. The official report said he died of his injuries, but Mark Lowenthal, a doctor who helped treat Julian in Ndola, told Björkdahl he could have been saved.

"I look upon the episode as having been one of my most egregious professional failures in what has become a long career," Lowenthal wrote in an email. "I must first ask why did the US authorities not at once set out to help/rescue one of their own? Why did I not think of this at the time? Why did I not try to contact US authorities to say, 'Send urgently an aircraft to evacuate a US citizen on secondment to UN who is dying of kidney failure?'"

Julian was left in Ndola for five days. Before he died, he told police he had seen sparks in the sky and an explosion before the crash.

Björkdahl also raises questions about why the DC6 was made to circle outside Ndola. The official report claims there was no tape recorder in the air traffic control tower, despite the fact that its equipment was new. The air traffic control report of the crash was not filed until 33 hours afterwards.

According to records of the events of the night, the British high commissioner to the Rhodesian and Nyasaland Federation, Cuthbert Alport, who was at the airport that evening, "suddenly said that he had heard that Hammarskjöld had changed his mind and intended to fly somewhere else. The airport manager therefore didn't send out any emergency alert and everyone simply went to bed."

The witness accounts of another plane are consistent with other insider accounts of Hammarskjold's death. Two of his top aides, Conor Cruise O'Brien and George Ivan Smith, both became convinced that the secretary general had been shot down by mercenaries working for European industrialists in Katanga. They also believed that the British helped cover up the shooting. In 1992, the two published a letter in the Guardian spelling out their theory. Suspicion of British intentions is a recurring theme of the correspondence Björkdahl has examined from the days before Hammarskjöld's death.

Formally, the UK backed the UN mission, but, privately, the secretary general and his aides believed British officials were obstructing peace moves, possibly as a result of mining interests and sympathies with the white colonists on the Katanga side.

On the morning of 13 September the separatist leader Moise Tshombe signalled that he was ready for a truce, but changed his mind after a one-hour meeting with the UK consul in Katanga, Denzil Dunnett.

There is no doubt that at the time of his death Hammarskjöld‚ who had already alienated the Soviets, French and Belgians, had also angered the Americans and the British with his decision to launch Operation Morthor against the rebel leaders and mercenaries in Katanga.

The US secretary of state, Dean Rusk, told one of the secretary general's aides that President Kennedy was "extremely upset" and was threatening to withdraw support from the UN. The UK , Rusk said, was "equally upset".

At the end of his investigation Björkdahl is still not sure who killed Hammarskjöld, but he is fairly certain why he was killed: "It's clear there were a lot of circumstances pointing to possible involvement by western powers. 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.

"Dag Hammarskjöld was trying to stick to the UN charter and the rules of international law. I have the impression from his telegrams and his private letters that he was disgusted by the behaviour of the big powers."

Historians at the Foreign Office said they could not comment. British officials believe that, at this late date, no amount of research would conclusively prove or disprove what they see as conspiracy theories that have always surrounded Hammarskjöld's death.