The trail for the killer of the Swedish prime minister had gone cold until a diplomat picked up the 1986 case as a hobby.

Dag Hammarskjöld brought me to Olof Palme. Two Swedish leaders, both supporting small nations on the world scene, both of whom refused to be controlled by global superpowers; both died a violent death. Were they also victims of the same forces?

For 11 years I investigated the mysterious aeroplane crash that killed the former UN secretary-general Dag Hammarskjöld, a a project that became the subject of the documentary Cold Case Hammarskjöld.

It was during a 2014 research trip for the documentary that the murder case of the former prime minister Olof Palme literally fell into my lap, when the journalist De Wet Potgieter passed me the so-called Deepsearch papers at the end of a dinner. The documents, prepared by the late general Tai Minnaar , describe how South Africa named Palme an “enemy of the state” and provide names of people allegedly involved in decision making, planning and implementation of his assassination.

Later I heard that these documents were viewed by many in Sweden as forgeries. But the next year, I met retired general Chris Thirion, a former head of South Africa’s Military Intelligence (MI), in Pretoria – and he told me on camera that documents in the Deepsearch papers looked genuine. He also said that he himself was convinced that South Africa carried out the assassination.

Through the Hammarskjöld investigation I had built an extensive network of contacts: former intelligence operatives, ex-military staff, historians, and journalists. For the Palme case, one of the most useful contacts was a former major general, Tienie Groenewald, who at the time of Palme’s murder had been in charge of South Africa’s National Intelligence Interpretation Branch. He told me fascinating stories of how South African military intelligence collaborated with the CIA, how the Israelis helped South Africa acquire the nuclear bomb – but not much of value for the Palme case.

After our last meeting, I called him and said that he, more than anyone, was in a position to help solve the Palme case. I mentioned the 50 million Swedish krona (£4.25m) reward and presented the idea of a deal in which Sweden would give immunity from prosecution and South Africa would convince any of its citizens involved in the assassination to come clean.

The next day, an employee of South African military intelligence called me and invited me to a meeting with a general in the covert section (I will not name him but later verified his identity).

I was instructed to go to the Hyatt hotel in Sandton, Johannesburg on 1 October, 2015. From there I was escorted to an almost empty restaurant nearby where I met the general.

He went straight to the heart of the matter, giving me the names of those he said were involved in Palme’s death – and told me that South Africa was willing to help Sweden get to the truth.

In return, South Africa was hoping for stronger relations with Sweden. A condition for such a deal would be immunity from prosecution for all those who acted on the orders of the former South African government.

He hinted that the motive could have been both political (Palme and Sweden supported the ANC) but also economic, although he didn’t elaborate. He said South African military intelligence was willing to start a discussion, but only with its Swedish counterparts – politicians and the media were to be excluded from the dialogue.

I said that the whole point of the initiative would be to finally go public so that the Palme family and the Swedish people could get closure. After an intense discussion, we agreed to let the intelligence agencies initiate the discussion and then take it from there.

The general was astonished that I was investigating the Palme murder as a hobby. He stressed that I was taking huge risks and that the operators would simply kill me if they felt threatened.

In November 2015, I handed over the material to Swedish national intelligence (Säpo), which in turn passed it to the police unit investigating Palme’s death.

For the next two and a half years, I heard nothing more, and in April 2018, I sought a meeting with the new prosecutor for the Palme case, Krister Petersson. I told him my story and was pleased to note that he already knew about it.

Not only that, Petersson asked me to let the MI general know that he was willing to go to South Africa and meet him.

I was never able to re-establish contact with the South African general but in June 2018 I succeeded in delivering the message to MI through another contact. Again, there was silence – until a few weeks ago, when an intelligence source in South Africa informed me that a meeting between representatives from the two governments had taken place in Pretoria on 18 March to discuss the Palme case.

In an ordinary legal case in Sweden, immunity against prosecution would be unthinkable. But the Olof Palme assassination case is unique

That’s why I hope the Swedish authorities can make an exception – provided of course that those responsible for the assassination come forward. I think it would be a respectable gesture by them to help the Swedish people get closure.

I think the need to know is greater than the need to punish.