Hammarskjöld was killed on the night of September 17-18, 1961, when his DC-6 came down near Ndola airport in the British colony Northern Rhodesia (modern-day Zambia) as he flew in to peace talks to end fighting in the mineral-rich Katanga province in neighbouring Congo.
The United Nations' own inquiry shortly after the crash blamed pilot error, effectively endorsing an initial investigation by the colonial authorities.
But reports from witnesses and other diplomats have led to widespread accusations of a cover-up and the suspicion that Hammarskjöld paid the price for supporting efforts by the newly independent Congolese government to crush an uprising in Katanga being funded by Western mining companies.
Göran Björkdahl, a Swedish aid worker who has interviewed eyewitnesses to the crash in Ndola, recently proclaimed that there could be "no doubt Dag Hammarskjöld's plane was brought down."
"My own conclusion, after adding the new witnesses' statements and the archive information to previously published documents, is that Hammarskjöld 's DC-6 was brought down and that the motive was to maintain the west's control over Katanga minerals," he wrote in Britain's Guardian newspaper last month.
A new book by the author Susan Williams entitled 'Who Killed Hammarskjöld?' also argues that the plane was brought down, prompting the diplomat's nephew Knut Hammarskjöld to recently call for a new inquiry.
Others who believe that the Swedish authorities have been too ready to accept the official version of events include Rolf Rembe who is the Swedish author of a book on Hammarskjöld 's career and legacy.
"What is important is to require all the relevant documents," Rembe told Sweden's daily Svenska Dagbladet.
"It seems unlikely, but it could have been an attempted kidnapping of Hammarskjöld," he added.
In 2007, retired French diplomat Claude de Kemoularia added fresh fuel to the conspiracy theory when he recounted a meeting with a group of mercenaries and a Belgian pilot in Paris in 1967 who claimed to have brought down the plane.
He then made contact with the Swedish authorities in the hope that they would open a new inquiry but his efforts were in vain. The foreign ministry later dismissed his version of events as "extremely unlikely."
John F Kennedy, the US president at the time of Hammarskjöld 's death, eulogised him as "the greatest statesman of our century."
In an address from the White House, Kennedy paid tribute to Hammarskjöld 's "dedication to the cause of peace, his untiring labor to achieve it, his courage under attack, (and) willingness to accept all responsibility in trying to strenghten the United Nations to make it a more effective instrument."
But Hammarskjöld was far from universally popular and Kennedy's Soviet counterpart Nikita Khrushchev had called for his resignation over his handling of the Katanga conflict.
Hammarskjöld responded: "It is very easy to bow to the wish of a big power, it is another matter to resist."
Hammarskjöld, who was 56 at the time of his death, is the only UN secretary general to have died while in office and also the only person to have been posthumously awarded the Nobel Peace Prize.
The current Swedish Foreign Minister Carl Bildt will travel on Sunday to Ndola, near the scene of the crash, to take part in a commemorative ceremony.
Sweden's Crown Princess Victoria will attend a similar event at the University of Uppsala and Swedish public radio's symphony orchestra will play a concert in his memory in the capital Stockholm.