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During World War II, Schäfer was head of the Nazi security police in Serbia. Between January and May 1942, Schäfer supervised the murder by gassing of around 7,300 Jews from the Semlin camp across the Sava river from Belgrade. A Saurer gas van was used to kill the 7,300. A further 1200 Jews died as a result of the camp's harsh conditions, or from executions. The van was used for the last time on 10 May 1942. In May 1942, Schäfer sent a cable to the Reich Main Security Office boasting "with pride" that "Belgrade was the only great city in Europe that was free of Jews."
In Germany after the war, Schäfer was sentenced to six and a half years in prison for his crimes during the war. He died in 1974.
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