Oskar Groening, 94, sat impassively as judge Franz Kompisch said "the defendant is found guilty of accessory to murder in 300,000 legally connected cases" of deported Jews who were sent to the gas chambers in 1944.
Groening served as a bookkeeper at the death camp in Nazi-occupied Poland, sorting and counting the money taken from those killed or used as slave labour, collecting cash in different European currencies, and shipping it back to his Nazi bosses in Berlin.
Groening had yesterday seized a last opportunity to address the judges and said he was "very sorry" for his time stationed at the concentration camp, telling them that "no one should have taken part in Auschwitz".
"I know that. I sincerely regret not having lived up to this realisation earlier and more consistently. I am very sorry," he said, his voice wavering.
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A group of Holocaust survivors said in a statement released after the verdict "we welcome the conviction of Oskar Groening", calling it a "very late step toward justice".
Groening has acknowledged "moral guilt" but said it is up to the court to rule on his legal culpability seven decades after the Holocaust.
Some 1.1 million people, most of them European Jews, perished between 1940 and 1945 in the Auschwitz-Birkenau camp before it was liberated by Soviet forces.