He watched fellow SS men kill children with their bare hands and saw Jews herded into the gas chambers of Auschwitz, but former death camp guard Oskar Groning has always denied any responsibility for the genocide of the Nazi Holocaust.

The trial of the 93-year-old "bookkeeper of Auschwitz" began overnight in the German town of Luneburg. Charged with complicity in the murder of 300,000 prisoners, Groning's case could set a precedent for other Holocaust convictions.

Unlike most war crime suspects, Groning declared he is ready to speak about the horrors of Auschwitz, where he was a guard from 1942 to 1944. "I have never found inner peace," he said a decade ago.

Groning is one of a handful of former Nazi death camp guards German authorities are struggling to put on trial before they die.

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In Auschwitz, he says he sorted money and valuables stolen from Jews before they were gassed, but denies a role in the mass murder.

"I would describe my role as a small cog in the gears. If you can describe this as guilt then I am guilty. Legally speaking I am innocent," he has said. But Groning also admits being a passive witness to appalling crimes against humanity. He saw an SS colleague murder a baby: "The crying bothered him, so he smashed the baby's head against the side of a truck until it was dead," he told Der Spiegel in 2005.

Prosecutors will rely on a legal precedent set by former Ukrainian-born John Demjanjuk, who was a guard at the Nazi extermination camp of Sobibor. Demjanjuk was convicted of complicity in mass murder by a Munich court in 2012. Judges ruled that merely by working at the camp, he was an accessory to mass murder.