BOGDANOVKA, Ukraine — From the porch of her mud hut, Vera Filonok saw tens of thousands of Jews shot, thrown in a ravine, and set on fire. Many were still alive, and they writhed in the flames “like flies and worms.”

The memories of what she saw in 1941 have seared her soul for six decades, but until recently, she had talked about it with no one except neighbors in her remote Ukrainian village. Then a soft-spoken French priest came to town.

Father Patrick Desbois and his small team of investigators have spent six years canvassing the towns and villages of Ukraine to patiently hear elderly people tell of what they saw during those terrible years when they were young. He says his team has pinpointed more than 600 mass execution sites, about 70% of them previously unknown. It has surveyed about a third of Ukraine, he says, and estimates there are at least 2,500 such sites throughout the Texas-sized country.

The work of Father Desbois and his Yahad-In Unum group is adding important new information to the history of the Holocaust — a period exhaustively studied in some countries but still veiled in much of the former Soviet Union. With the Soviet collapse, the declassification of Soviet war archives, and the general opening up of this country of 47 million, it has now become possible to speak to the witnesses.

Vital to the effort, Father Desbois said, is the U.S. Holocaust Memorial Museum in Washington and its vast Soviet archival material available. That and Father Desbois’s field work have expanded historians’ knowledge about the public nature of the killings, the large variety of methods of execution, and the Nazis’ forced recruitment of children to assist in their actions.

“You have a marriage of validation with the sources 60 years apart,” said Paul Shapiro, director of the Center for Advanced Holocaust Studies at the U.S. Holocaust Memorial Museum in Washington. “Using the two sources together one can understand what happened on the ground in those towns and villages in Ukraine.”

While the Soviet Union glorified its victory over the Nazis, it refused to acknowledge the massive and systematic killings of Jews. The refusal reflected both anti-Semitism and official resistance to singling out ethnic groups in what was supposed to be a single Soviet nation.

Historians say some 1.4 million of Soviet Ukraine’s 2.4 million Jews were executed, starved to death, or died of disease during the war. Their remains are strewn around the country in common graves, many of them ignored and unmarked.

The destruction of Ukrainian Jewry is epitomized by Babi Yar, a ravine in the capital Kiev where the Nazis killed about 34,000 Jews during just two days in September 1941. What happened in Bogdanovka was even bloodier — 48,000 killed. And the perpetrators were not Germans but Romanians and the Ukrainian police they enlisted to help them play their own part in Hitler’s genocide.

In 1941, Romanian troops allied with the Nazis occupied a large part of what is now southern Ukraine and began exterminating Jews. Those who survived the initial killings were herded to Bogdanovka and placed in stables and pigsties teeming with fleas and manure.

The massacre began when the Romanians and Ukrainians nailed shut one of the pigsties’ doors and windows, then torched it, burning all inside alive. The killing went on for three weeks in late December 1941 and early January 1942 — with a break for Christmas.

Vera Filonok was 16 when she witnessed the blaze from Konstantinovka, a village lying across the quiet Bug River. “We sensed the smell — of burning hair, clothes, bones — a very strong, acrid smell,” she said, raising a hand to her wrinkled face. “People were being burned alive. For me, that was the most terrifying thing.”

After the fire came gunshots, recalled Ms. Filonok’s neighbor Raya Trofimova. A German soldier living in her family’s home lent her his binoculars; through them she saw victims kneel in front of a gully in their underwear, their valuables piled beside them. “They would line them up before the ravine and shoot them … they would tear away children from mothers and just throw them in there,” said Ms. Trofimova, now 85. When her mother returned home that day, Trofimova recalled, she shut the windows and draped them with blankets to shield her three children from the sights and smells.

“Ta-ta-ta,” Ms. Trofimova mimicked the gunshots ringing out across the river, day and night. “And what could we do? If you protested, you would be taken to the same pit.”

Anatoly Veliminchuk was 11. He said he saw people thrown into two wells, many still alive.

“I felt bad, it was painful — what did it matter that they spoke their Jewish way and we spoke Ukrainian or Moldovan?” he asked as he pointed to what used to be the wells — now two small pits in a field covered with dry grass and discarded plastic bottles.

Father Desbois registers an event or killing site only after obtaining three independent witness accounts. His team has two translators, a photographer, and cameraman, a ballistics specialist and a mapping expert.

The 52-year-old priest was raised on his grandfather’s stories of surviving a Nazi prison camp in Ukraine, and has devoted his career to healing wounds between Catholics and Jews. His group, Yahad-In Unum — which combines the Hebrew and Latin words for “together” — was founded by influential Catholics and Jews.

Ukraine’s Jewish leaders say the community is grateful for the effort. “What they are doing is holy work, because everybody is forgetting about this tragedy,” Yakov Blaikh, Ukraine’s chief rabbi, said. The Holocaust is still controversial and divisive in Ukraine because of the wartime collaboration with the Nazis, and the museum nearest to Bogdanovka commemorates those “who saved the motherland,” but says nothing about the massacres of Jews.

Anatoly Podolsky, head of the Ukrainian Center for Holocaust Studies, said Ukraine still needs to more fully confront the Holocaust, but that allowing Desbois to operate here shows that “There is no longer that endless untruthful silence that existed in Soviet Ukraine.”

Shattering that silence is Father Desbois’s goal.

At the end of a long day of talking with tearful witnesses, his shoes covered with dust, Father Desbois said his mission isn’t to seek retribution — “I am not here to judge” — but to record the tragedy for the sake of its victims.

“God wants these poor people to be finally buried and rest in peace … and that they receive the Jewish prayer they deserve,” he said.