The total number killed at Grini is unknown, though the Gestapo and police often used the area for purposes of torture and at least eight people were executed there. British airborne troops sent by glider to sabotage the Norsk Hydro heavy-water plant during Operation Freshman crashed in Norway due to foul weather. The five uninjured survivors were taken prisoner and held at Grini concentration camp until 18 January 1943, when they were taken to nearby woods, blindfolded and shot in the back of the head by the Gestapo. This was a war crime, in breach of the Geneva Convention. Executions normally took place at Akershus Fortress or Trandumskogen.