 Harnack was arrested in 1933, after he had tried to determine the murderers of Mecklenburg-Schwerin's former Prime Minister and SPD politician Johannes Stelling, murdered during the so-called Night of the Long Knives.In co-operation with Willi Wohlberedt, he created a graves registration index cards for Berlin.Accused of being a participant in the 20 July Plot in 1944, he was executed in March 1945 in Plötzensee and buried in an unmarked grave.