Exactly 50 years ago, UN Secretary General Dag Hammarskjold died in a plane crash on a mission to prevent civil war in newly independent Congo. Suspicions that the plane was shot down, never fully laid to rest, are now again on the rise.

After his death, Mr Hammarskjold was described by US President John F Kennedy as the "greatest statesman of our century". He was a man with a vision of the UN as a "dynamic instrument" organising the world community, a protector of small nations, independent of the major powers, acting only in the interests of peace.

Dag Hammarskjold Born in 1905 into an aristocratic Swedish family

Full name, Dag Hjalmar Agne Carl Hammarskjold

Helped lay foundations of Swedish welfare state

Swedish state secretary for foreign affairs (1947-1951)

The UN's second secretary general (1953-1961), proposed by Britain and France

Nobel Peace Prize winner 1961

The only person to be awarded a Nobel Peace Prize after his death, he established the first armed UN peacekeeping mission following the crisis in Suez.

Just after midnight on 18 September 1961, he was heading to negotiate a ceasefire in a mineral-rich breakaway region of Congo, where another of his peacekeeping missions was getting bogged down in the complex politics of decolonisation and Cold War rivalry.

But his DC6 aircraft crashed in darkness shortly before landing, in a forest near Ndola in Northern Rhodesia - now Zambia.

Knut Hammarskjold, his nephew, visited the crash site days later.

"It was just scattered all over the place, the pieces of the aircraft," he says. "I did not see any bodies, they had been removed earlier, I think."

He remembers the reaction at home in Sweden, where his uncle was a national hero.

"Everybody was so shocked. I can say the whole of Sweden was affected by this. All the shops had his picture in the window, and he had a state funeral which was very unusual for a foreign office person."

Iron will

There is a mass of evidence that points in the direction that the plane was shot down by a second plane Susan Williams, Author of Who Killed Hammarskjold?

Eight years earlier, when the members of the Security Council appointed the unassuming Swede secretary general, they could not have predicted the zeal he would bring to the job.

"He was a very spiritual, intellectual, cultured man, and that was all part of his mystical approach to life," says Dame Margaret Anstee, the first female under-secretary at the UN, who was starting out on a 40-year career at the organisation. "He had a certain reserve, and a certain unique kind of dignity."

But he soon gained a reputation for independence and daring, and instead of staying in his New York office, a hands-on approach became his trademark. He personally negotiated the release of 15 American airmen who had been imprisoned in China at a time when the People's Republic was not represented at the UN.

"He had the skills of mediation and persuasion, combined with this almost iron single-minded will of where he wanted to go," says Margaret Anstee.

"But of course by that very token it brought him into conflict with people who wanted to use the UN for their own ends."

In Congo, one issue was who should control the southern province of Katanga, rich in copper, uranium and tin. Belgium, the ex-colonial power, backed a secessionist movement led by Moise Tshombe, as did the UK and US who had mining interests in the region.

Some Unanswered Questions Why was a search and rescue operation not launched immediately, or at dawn?

Why did authorities only arrive, officially, at the crash site after 15:00 on the 18th?

Why of all those on board was Dag Hammarskjold's the only body undamaged by fire?

Why was a playing card, said to be the ace of spades, apparently found tucked into his collar?

But Mr Hammarskjold from the start backed Congo's elected central authorities - the Soviet-backed government of prime minister Patrice Lumumba, and later, after Mr Lumumba was deposed and murdered, Prime Minister Cyrille Adoula.

Mr Hammarskjold wanted to pursue a negotiated solution between Mr Tshombe and the central government, a goal that became even more urgent after UN peacekeepers found themselves outgunned during an aggressive operation to drive foreign mercenaries from Katanga.

Mr Tshombe was waiting to talk to him in Ndola on the night he died.

Airbrushed photos

The crash of his aircraft has never been fully explained. Two investigations held in the British-run Central African Federation, which included Northern Rhodesia, were followed by an official UN inquiry, which concluded that foul play could not be ruled out. So people have never stopped coming forward with new explanations, and asking new questions.

Image caption The crash scene, photographed by Katanga UN representative, George Ivan Smith

Some 30 years after the crash, in 1992, two men who had served as UN representatives in Katanga just before and just after Hammarskjold's death - Conor Cruise O'Brien and George Ivan Smith - wrote a letter to the Guardian claiming to have evidence that the plane was shot down accidentally, by mercenaries. In their view, a warning shot intended to divert the plane to alternative talks with industrialists in Katanga, in fact hit the plane and caused it to crash.

In 1998 South Africa's Truth and Reconciliation Commission, led by Desmond Tutu, published eight letters that suggested CIA, MI5 and South African intelligence were involved in sabotage of the aircraft. British officials responded that these were likely to be Soviet forgeries.

In 2005, the head of UN military information in Congo in 1961, Bjorn Egge, told the Aftenposten newspaper he had noticed a round hole in Hammarskjold's forehead when he saw the body in the mortuary. It could have been a bullet hole, he said, and it had been mysteriously airbrushed out of official photographs.

Over the past four years, Swedish aid worker Goran Bjorkdahl has carried out extensive research and British academic Susan Williams published a book on Thursday - Who Killed Hammarskjold? Both conclude that it is likely the plane was brought down.

Mr Bjorkdahl began his study after inheriting from his father, who had worked in Zambia in the 1970s, a piece of the plane fuselage containing unexplained small holes. He tracked down 12 witnesses, in whose accounts of the night three points appeared repeatedly:

The DC6 circled in the air two or three times before it crashed

A smaller plane flew above it

A bright light flashed in the sky above the large plane before it went down

Six witnesses also recall seeing uniformed personnel near the crash site that morning, even though official reports claim it was not located until after 15:00 that day.

The official inquiries held at the time also contain witness testimony referring to a second plane in the sky.

One of the key questions Ms Williams asks in her book is why this and other inconvenient observations were discounted, or in some cases doctored during the official Rhodesian investigations. She says it is clear to her that there was a cover-up.

She places particular emphasis on three of her discoveries:

The photographs of Hammarskjold after his death are either taken in such a way as to conceal the area around his right eye, or, where the eye is visible, they show evidence of having been touched up, possibly to hide a wound

The sole survivor of the crash, Harold Julien, said there was an explosion before the plane fell from the sky - his evidence was discounted in the original inquiry on the grounds that he was ill and sedated, but Ms Williams has found a doctor's statement insisting that he was lucid at the time (he died of his injuries within days)

A US intelligence officer at a listening station in Cyprus says he heard a cockpit recording from Ndola, in which a pilot talks of closing in on the DC6 - guns are heard firing, and then the words "I've hit it"

"There is no smoking gun, but there is a mass of evidence that points in the direction that the plane was shot down by a second plane," she told the BBC. "That is a far more convincing and supported explanation than any other."

There were a range of people, including white Rhodesians and the Belgian and British mining companies in Katanga, "with a sense of being at war with the UN and with African nationalism", she says - and with a motive for preventing Mr Hammarskjold and Mr Tshombe reaching a negotiated settlement.

Model diplomat

Mr Hammarskjold's main adviser at the time, Brian Urquhart, says it is "so wrong" to think that "at night without ground control you could shoot down a plane or even locate it". But Ms Williams says experts have told her that the DC6, on its way in to land at Ndola airport on a moonlit night, was a "sitting duck".

There was a tacit agreement never to have such a single-minded Secretary General again Dame Margaret Anstee

Ms Williams argues that the time has come for a new inquiry, and Mr Hammarskjold's nephew Knut is reported to have called for one himself, after hearing of Ms Williams' new evidence.

Fifty years later, his uncle is still a model for people working at the UN, says Knut Hammarskjold.

"Many, I've been told, still have his photo on their desks, and [former Secretary General] Kofi Annan says he always asks when there is a problem: 'What will Dag have done in this situation?'"

Dame Margaret Anstee says he had the courage to stand up for his principles and to the strong member states, which his successors have lacked.

"There was a tacit agreement never to have such a single-minded secretary general again," she says. "I think we can say they haven't."

Additional reporting by Stephen Mulvey

Susan Williams' book, Who Killed Hammarskjold?, is published by Hurst and Company.

BBC World Service's Witness programme on Monday reports on Dag Hammarskjold's life and death, featuring contributions from Knut Hammarskjold and Dame Margaret Anstee