In July 2022 former Iranian official Hamid Nouri was convicted by a Swedish court of murder and war crimes and sentenced to life imprisonment. His offences were committed in Iran's prisons in 1988, when an estimated 5000 political prisoners were killed. For the survivors and the families of those who died it was a crucial breakthrough in their long campaign for justice. BBC Persian's Omid Montazeri, whose own father was killed for his communist beliefs in the massacres, follows the trial for Our World and tells the story of how a killer was caught.