Poland was the site of some of the most notorious Nazi German death camps, including Auschwitz-Birkenau, Treblinka and Sobibor.

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In March this year Ukrainian-born John Demjanjuk, found guilty for his role as a Nazi guard at the Sobibor death camp, died aged 91.

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They said they had documents proving his Nazi background, including an SS identity card which showed he had been a guard at Sobibor between March and September 1943, and many witness testimonies.

BBC: Demjanjuk trial set for November

But the case is now being pursued on the same legal theory used to prosecute former Ohio autoworker John Demjanjuk, who died last year while appealing his 2011 conviction in Germany for accessory to murder on the grounds that he served as a guard at the Sobibor death camp.

WSJ: Germany Arrests Man Who Worked at Auschwitz