Jakobs' court martial took place in front of a military tribunal at the Duke of York's Headquarters in Chelsea, London SW3, on 4–5 August 1941. The trial was held "in camera" because the German agent had been apprehended in a highly classified intelligence operation known as the Double Cross System. The British were aware that Jakobs was coming because his arrival information had been passed on to MI5 by the Welsh nationalist and "Abwehr" double agent Arthur Owens. After a two-day trial which involved hearing the testimony of eight witnesses, Jakobs was found guilty of spying and sentenced to death.