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He was an Oberregierungsrat and held the rank of SS-Obersturmbannführer (Lieutenant Colonel). He was born in Posen, Province of Posen, German Empire. He became a member of NSDAP in 1933. In 1934, he took his degree in law and in 1936 was appointed by Reinhard Heydrich to the Gestapo where he was appointed to controlling the religious sects of Germany. In 1938, he took part in the German march into the Sudetenland, and in 1939, in the annexation of Bohemia and Moravia, and was later appointed political adviser to Gauleiter Fritz Sauckel in Thuringia. After the outbreak of the war in September, 1939, he became leader of an Einsatzkommando in Poznań (German: Posen). In 1940, he joined the 3rd SS Division Totenkopf in their march into France.
He had a position as Regierungsrat (Executive Council, government advisor), and SS-Sturmbannführer (Major), in April 1940 when he was assigned to Norway. His first job in Norway was Kommandeur der Sipo und des SD in Bergen (The Sicherheitspolizei (security police), often abbreviated as SiPo, was a term used in Nazi Germany to describe the state political and criminal investigation security agencies. It was made up by the combined forces of the Gestapo (secret state police) and the Kripo (criminal police) between 1934 and 1939. As a formal agency, the SiPo was folded into the RSHA in 1939, but the term continued to be used informally until the end of the Third Reich). On 11 October 1941, he was appointed Kommandeur der Sicherheitspolizei und des SD (the Sicherheitsdienst (SD, Security Service) was primarily the intelligence service of the SS and the Nazi Party in Nazi Germany), in Trondheim. As Kommandeur of the district, he was also chief of Falstad concentration camp outside Trondheim and the prisons in Trondheim. He was given the rank of Obersturmbannführer (Lieutenant Colonel) and received the title of Oberregierungsrat. His immediate superior was Heinrich Fehlis. On 8 May 1945, he fled from Trondheim with a gold bar in his luggage. He was caught and sent back with a police escort on the train and during which he made an unsuccessful attempt to escape.
He was known for being a notorious torturer, and ordered the execution of many members of the Norwegian resistance movement without any trial. After World War II, in 1946, he was tried and sentenced, for many cases of torture and murder, to execution by firing squad. The sentence was carried out at midnight at Kristiansten festning on 28 February 1948. Right before the order was given to fire, Gerhard Flesch shouted loudly "Heil Hitler".