Germany invaded Greece in the spring of 1941 and in February 1943 confined the Jews to two ghettos. Deportations to Auschwitz began shortly after. Mr. Gabbai and his family were among the last rounded up and, in April 1944, were sent on an 11-day journey crammed in cattle cars with little food and no toilet.

Upon arrival at Auschwitz, Dario saw his father, mother and youngest brother dispatched to the gas chambers. (A sister had died as an infant, before Auschwitz.) Athletic and muscular at 21, he was selected, along with his older brother, Jakob, as Sonderkommandos at nearby Birkenau.

Soon he was disposing of the corpses of newly arrived Hungarian Jews. More than 6,000 were killed each day, according to Mr. Berenbaum, a professor of Jewish studies at American Jewish University in Los Angeles.

In the few minutes that the gas chamber doors were locked, Mr. Gabbai could hear the hundreds of women and children screaming, crying and scratching the walls in desperate efforts to breathe. When the doors opened, he and other Sonderkommandos had to climb over bodies piled five and six feet high to harvest glasses, gold teeth and prosthetic limbs, before hauling out the corpses and washing down floors and walls covered in blood and excrement.

Mr. Gabbai also recalled being handed scissors and ordered to cut the hair off the women, so it could be used to make blankets and socks for German soldiers. When a sound emerged from a corpse, he said, he was so sickened that he said to himself, “Where is God?”

“I saw the people I just saw alive, the mother with the kids in their arms, some black and blue from the gas, dead,” Mr. Gabbai said. “I said to myself — my mind went blind, how can I survive in this environment?”

Mr. Gabbai and other Sonderkommandos would then drag the bodies to an elevator that would lift them one flight to the furnace floor. There was also a dissecting room, where jewels and other valuables hidden in body crevices would be removed. To endure such grim work, Mr. Gabbai told friends, he “shut down” and became an “automaton.”