PARIS, Oct. 5 — His subjects were mostly children and teenagers at the time, terrified witnesses to mass slaughter. Some were forced to work at the bottom rung of the Nazi killing machine — as diggers of mass graves, cooks who fed Nazi soldiers and seamstresses who mended clothes stripped from the Jews before execution.

They live today in rural poverty, many without running water or heat, nearing the end of their lives. So Patrick Desbois has been quietly seeking them out, roaming the back roads and forgotten fields of Ukraine, hearing their stories and searching for the unmarked common graves. He knows that they are an unparalleled source to document the murder of the 1.5 million Jews of Ukraine, shot dead and buried throughout the country.

He is neither a historian nor an archaeologist, but a French Roman Catholic priest. And his most powerful tools are his matter-of-fact style — and his clerical collar.

The Nazis killed nearly 1.5 million Jews in Ukraine after their invasion of the Soviet Union in June 1941. But with few exceptions, most notably the 1941 slaughter of nearly 34,000 Jews in the Babi Yar ravine in Kiev, much of that history has gone untold.