As a child in post-war France, growing up in a secular Catholic family, Patrick Desbois learned that his paternal grandfather, Claudius Desbois, a captured French soldier during World War II, had been a POW in a Nazi work camp at Rawa Ruska, on the Ukrainian border near the current Polish border.

Claudius, “a joker,” would turn silent when the subject of Rawa Ruska came up, his grandson remembers.

“We were locked in a camp with nothing to drink, we ate grass and dandelions, but outside the camp, for the others, it was worse,” Claudius would tell his grandson.

Where was “outside,” young Patrick wondered. Who were “the others”? What was “worse”?

He determined to answer those questions.

Desbois, now 62, a “militant atheist” who found God during college and embraced his Catholic heritage and became a priest, has dedicated the last few decades of his life to that mission.

In archives and research trips to the sites of Nazi killings, he found that the “outside” was concentration camps and the killing fields of Eastern Europe, where Jews and other victims of the Third Reich lost their lives in conditions of unspeakable cruelty.

Father Desbois first documented his research in “The Holocaust by Bullets: A Priest’s Journey to Uncover the Truth Behind the Murder of 1.5 Million Jews” (Palgrave Macmillan, 2008). The book described the work of the Einsatzgruppen, the German Army’s mobile killing squads that swept through the occupied lands between Galicia and the shores of the Baltic Sea, between the forests of Moscow and the Caucasian borders, killing the Jewish inhabitants of village after village, often in empty fields or ravines on the edge of town.

A self-trained forensic anthropologist, he located the mass graves where more than a million Jews are buried.

And now, a decade later, he has just published “In Broad Daylight: The Secret Procedures behind the Holocaust by Bullets” (Arcade Publishing). In his new book, which includes interviews with witnesses conducted for “The Holocaust by Bullets” and subsequent research, Father Desbois tells the story of the men and women, most of them children during WWII, who watched, and often assisted — willingly or at gunpoint — the Nazi killing machine at work.

The aging witnesses he tracked down welcomed the priest and his film crew into their usually modest homes, he says one recent morning in New York, where he’d come to give a speech and promote his book.

“Why did you come so late?” they would ask him — why hadn’t anyone asked before about what they had seen 75 years ago?

Their memories still clear, they would share their stories and take Father Desbois to the nearby sites where they had seen — or participated in — the Nazi-orchestrated murders.

Almost no one turned Father Desbois away, he says.

They weren’t confessing their guilt to a priest, but giving testimony to a historian, he says.

Father Desbois, who splits his time between Paris and Washington, where he holds an endowed professorship at Georgetown University, had predicted 10 years ago that all the aging witnesses would die out by now.

He was wrong, he says; there are still witnesses left, still mass graves to identify.

Later this year, his third book on a related, humanitarian theme will come out: “The Terrorist Factory: ISIS, the Yazidi Genocide, and Exporting Terror” (Arcade Publishing). That book, co-authored with Roma activist Costel Nastasie, deals with the murder in Iraq, by members of the self-proclaimed Islamic State, of members of the religious minority who have roots in Zoroastrianism.

What he heard from Yazidi survivors sounded familiar. ISIS killers, Father Desbois said, are the same as the Nazis whose atrocities he has documented.

“It was the same methodology,” the same type of racist ideology, he said.

To take up the cause of the Yazidi, Father Desbois established “Action Yazidis” as part of Yahad-In-Unum, his Paris-based human rights organization whose main focus has been the Nazis’ Final Solution.

He tells the stories in the bystanders’ and collaborators’ own words, offering details of how the non-Jewish residents of villages and farms in Ukraine and Belarus and other countries dug the mass graves, laid planks over them, fed the Nazi troops and performed countless other necessary-for-mass-murder functions.

“It took many pairs of hands — voluntary, requisitioned, or forced — to ensure that the Jews were publicly murdered,” Father Desbois writes. “Armed men are not sufficient to commit genocide. It requires so many arms, legs, shovels, and wagons — so much labor.”

Before Father Desbois pair of books about the Holocaust, no researcher had done that type of in-depth research about the mechanics of the Einsatzgruppen.

“What Father Desbois’ is doing is tremendous,” said Holocaust historian Michael Berenbaum. “No one else is doing the original groundwork. The evidence he has gathered is very important — the forensic evidence of how they did the killing by bullets.”

Why does Father Desbois, a Catholic, care so much about the fate of other religious groups?

He’s heard this question countless times before and is clearly uncomfortable analyzing his motives.

“I’m not complicated,” he says. It’s simply his holy mission. “God kept me alive to tell this story.”

Maybe his motivation goes back to what he heard as a child about his grandfather’s fate in Rawa Ruska, he offers.

What would his grandfather think about his work?

Father Desbois has heard this question many times too and is equally uncomfortable answering it.

He shrugs. He can’t say what his grandfather would think. “I don’t ask questions like this.”

steve@jewishweek.org