In 1961, the UN Secretary General Dag Hammarskjöld died when his plane crashed on the way to cease-fire negotiations during the Congo Crisis. Described as the “greatest statesman of our century” by John F. Kennedy, the Swede was posthumously awarded the Nobel Peace Prize for ‘strengthening the organisation’ and was considered to the template his successors aspired to. He was influential in a way no one subsequently is his role has been. The circumstances around his death are still disputed to this day.

Cold Case Hammarskjöld, the new documentary from Mads Brügger, re-visits the diplomat’s death from a fresh angle. Along with Göran Björkdahl, who is in possession of a metal plate which purports to belongs to the plane, he has a theory. They believe Dag Hammarskjöld was assassinated by shadowy forces due to his involvement in African, and therefore international, politics. Their investigation opens a Pandora’s Box and suggestions that there was a much larger and more troubling conspiracy.

As you watch Cold Case Hammarskjöld, you’re likely to find yourself increasingly troubled. Firstly, by some of the revelations which spill out, which if true would be on a par with humanity’s worst moments. Throughout the film, Brügger, keeps track of his investigations through conversations with his secretary. He uses her as a sounding board, to check his knowledge and in doing so cleverly keeps the audience up to speed. Secondly, it’s difficult to know how much of what the pair discover is actually true. Hard evidence is at a premium and their chief witness raises several red flags. Whilst Cold Case Hammarskjöld asks more questions than it answers it’s a fascinating and terrifying peek into a troubling history.

Cold Case Hammarskjöld screens again at London Film Festival on 5 October.