As Mr. Bialowitz and his family arrived with other Jews, they were asked if any had professions. Symcha said, truthfully, that he was a pharmacist, and, untruthfully, that his brother was his assistant. That saved their lives. Separated on the spot from their relatives, who were doomed, they were put to work as slave laborers, performing tasks that included collecting the clothing and valuables of their dead brethren.

Some months afterward, on Oct. 14, 1943, the Jewish workers carried out an escape plan, enticing guards to relax their vigilance by offering up a hoard of salvaged leather jackets and boots. Several SS officers were killed with knives and hatchets that the prisoners had secreted away, and about 300 fled through the gates of the camp. Nearly all were recaptured, but the Bialowitz brothers were among the 50 or so who were not. Symcha died at 102 in 2014 in Israel.

Shortly after the escape, the Nazis began to dismantle the Sobibor camp, one of three (along with Treblinka and Belzec) built for the explicit purpose of killing under the auspices of Operation Reinhard, the Nazi code name for the extermination of Jews. The history of the place was so well concealed that the underground remains of the gas chambers were not unearthed until 2014, news of which Mr. Bialowitz said “was the best moment of my life.”

“The Germans tried to do everything they could to hide their crime,” he told the German news website Spiegel Online. “But now archaeologists have discovered the gas chambers. Sobibor will become better known, and it will serve as an educational center for future generations. This is a victory — not only for us as survivors, but also for the whole mankind.”