In 1975 my father, then working for the UN in Zambia, visited the spot where Dag Hammarskjold's plane crashed outside Ndola. He talked about the crash to a worker there, Dickson Mwewa, who offered him a metal plate found buried at the site, telling him it was a piece of the crashed DC6. My father told the Swedish authorities about the plate at the time but they showed no interest. I discovered the plate 32 years later while helping my parents to move house. There were five strange holes in the metal. A forensics expert later told me they were too small for bullet holes, but he could not account for them. I have not managed to find any explanation in the four years since, but I have since become absorbed in the case, which remains one of the last century's great unsolved mysteries.

While on a mission to Zambia in 2007 I visited Ndola and met an old charcoal burner, Moses Chimema, who claimed to have seen the crash. His story intrigued me and I started to search for other witnesses. Together with my Zambian friend Jacob Phiri I went around in the bush, on bicycle and by car, asking for people who could have been there in 1961. After three years we ended up with a dozen witnesses — whose stories all match Chimema's but are contrary to the official inquiry reports. I filmed interviews with them.

Archive documents in Stockholm, Lusaka and Oxford painted a picture not only of the crash but also the events in Katanga and the political machinations that led to Hammarskjold's last journey. Interviewing the witnesses and reading the 50-year-old documents became a journey back to an era of cold war, struggle for liberation and fights over mineral fortunes.

The secret UN cables between UN headquarters in New York and the UN mission in Congo proved to be a mine of information.

They reveal the growing frustration of Hammarskjold and his officials over the tactics used by the powerful mining company Union Miniere, owned mainly by Belgian, British and American investors, to obstruct and undermine the UN mission in Congo.

The documents show how the UN came to draw up a joint plan with the country's central government, Operation Morthor, aimed at ending the Katanga secession by force and the subsequent fury of US president John Kennedy and the British prime minister Harold Macmillan, who felt they had not been consulted. Hammarskjold sent a robust reply and openly questioned the motives of the western powers in Congo. With Morthor failing, and under considerable pressure from Washington and London, Hammarskjold apparently felt compelled to deny the original UN objective had been to end the Katanga secession by military means, and he deliberately misled the security council with a false account of the hostilities.

The cables have been available at Sweden's Royal Library since 1994, but have never been published in full. Brian Urquhart refers to them in his 1972 biography of Hammarskjold, but misinterprets a critical cable on Sept 10, 1961, conveying Hammarskjold's approval of Morthor. Urquhart says that Hammarskjold had instructed his aides to consult him before taking any action. In fact, the secretary-general gives his men the green light. The Morthor fiasco was ultimately his responsibility.

The cables and also Hammarskjold's private letters depict a strong UN leader guided by the UN charter, with a strong sympathy for the emerging new nations — as well as a dislike of the big powers’ arrogance and hypocrisy. He won diplomatic victories over France and the UK in the Suez crisis in 1957 and over France in the Bizerte crisis in 1961 and he gave moral support to newly independent Guinea. After this, President Charles de Gaulle withdrew France's support for the UN Congo operation, boycotted the security council meetings and even encouraged French mercenaries to join the Katanga forces.

It is ironic that France and the UK were the ones who proposed Hammarskjold's candidature to the UN secretary-general post in 1953, probably seeing him as a skilled bureaucrat but politically weak. It is clear that none of the big powers wanted such a forceful UN leader, guided by principles and impossible to control.

My own conclusion, after adding the new witnesses’ statements and the archive information to previously published documents, is that Hammarskjold's DC6 was brought down and that the motive was to maintain the west's control over Katanga minerals. It is significant that the UN, after Hammarskjold's death, has become less of a challenge to the big powers. —Dawn/ Guardian News Service