A decision to investigate 30 alleged former guards at Auschwitz was prompted by the case of Ukraine-born John Demjanjuk.
Demjanjuk was the first Nazi war criminal to be convicted in Germany without evidence of a specific crime or victim but purely on the grounds he had served as death camp guard.
He died last year, aged 91, while appealing against a five-year jail sentence for complicity in the murder of more than 28,000 Jews at the Sobibor camp in Poland.
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The Duchess of Cambridge has been on maternity leave since the week before Easter.
The Duke and Duchess of Cambrige have been together for 15 years and will have three children under the age of four.
News of William and Kate’s third child was announced on September 4 and the duchess went on maternity leave on March 22.