A former officer in the East German secret police has been sentenced to 10 years in jail for the murder of a Polish firefighter at a Berlin border crossing 50 years ago.

Martin Naumann, now 80, shot Czesław Kukuczka in the back at close range on 29 March 1974 as Kukuczka walked towards the last in a series of control posts at a transit area in the divided city, having been told he had a free pass to escape to West Berlin.

The truth surrounding Kukuczka’s death was never revealed to his family. Instead, his cremated remains were sent in an urn to his wife, Emilia, weeks later, after which he was buried in a private ceremony by his family in southern Poland.

It took the dogged research skills of a historian immersed in the history of the Ministry for State Security (MfS), or the Stasi – which was the intelligence service and secret police of the communist GDR – to unearth the details of the case years later.