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Who is to blame for the killing of 1.5 million Jews in Nazi-occupied Ukraine? How do you define a wartime collaborator? And what can be done now to dispel age-old anti-Semitism in Ukraine, honor Jewish dead and move on?

For the first time, scholars from around the world shared documents and knowledge about the Holocaust in Ukraine at a conference this week in Paris dedicated to this poorly understood passage in Adolf Hitler's torrent of continent-wide terror.

The talks were not easy, as resentment, frustration and emotion bubbled repeatedly to the surface among the researchers from Israel, Ukraine, Germany, the United States and elsewhere.

While no major surprises emerged, pieces of Ukraine's Holocaust story came together as never before: massacres of Jews in western Ukraine even before the occupying Nazis arrived, botched Soviet orders to evacuate Jews from the encroaching Germans, mass grave sites only now being discovered — even as long-known Jewish grave sites are being abandoned, razed, used as open air markets.

"We cannot underestimate this. It is historic, it is history that ... may be changed based on new information," said Mikhail Tyaglyy of the Ukrainian Center for Holocaust Studies.

History books, too, many need to be changed — or written — to explain how an estimated 1.4 million of Ukraine's 2.4 million Jews disappeared in just three years, 1941-1944. After repeated waves of emigration, only about 100,000 remain today, according to official figures.

While the Holocaust has long been well-documented in western and central Europe, few have studied what happened when Nazi tentacles spread into what is now Ukraine, Belarus, Moldova and western Russia.

Soviet authorities discouraged such scholarship, content to keep history books focused on the costly Red Army victory over Hitler's forces. State-condoned suspicion of Jews, meanwhile, continued to proliferate as it had since the pogroms of pre-revolutionary times.

Tyaglyy long worked in near-isolation in his native Black Sea peninsula of Crimea, while colleagues worked in the capital Kiev or in Kharkiv in the east. Ukraine's central government has paid their studies little heed.

On Monday, they earned welcome recognition as they joined the dais at the University of Paris IV-Sorbonne for two days of often electric, sometimes heartrending talks. Related discussions were continuing at other venues in Paris throughout the week.

"This is two totally different universes coming together," that of troubled, long closed-off Ukraine and of well-equipped Western research organizations, said Father Patrick Desbois, a French Roman Catholic priest whose recent work gathering testimonies from Ukrainian Holocaust witnesses helped inspire the idea for the conference.

These testimonies, including those of destitute villagers who had rarely if ever spoken about what they saw and did during the war, formed the most powerful evidence presented. Some are on display at Paris' Holocaust Memorial.

They speak of methodical hailstorms of bullets; of fading cries rising up from pits of the dead and dying; and then silence, generations of silence only now being broken in a burst of shameful memories and questions with no easy answers.

The question of collaboration haunted this week's talks. Were the Ukrainian teenagers ordered to carry corpses and dig pits collaborators or unfortunate witnesses to the Holocaust? No consensus was reached.

Omer Bartov, of Brown University, said such testimonies are only one step in understanding the Holocaust in Ukraine and confronting today's disregard for Jewish heritage.

He showed photos of Jewish schools, synagogues and cemeteries crumbling into disrepair across western Ukraine.

"I urge more money spent on preserving what there is," he said. "That would be a good start for a better relationship."

He pointed to Poland as an example of a country that is coming to terms with its role in the Holocaust and profiting from renovating Jewish neighborhoods and reviving Jewish culture in cities such as Krakow.

Paul Shapiro, director of the Center for Advanced Holocaust Studies at the U.S. Holocaust Memorial Museum, urged Ukraine and the conference participants to focus next "not just on commemorating Jewish death but also on celebrating Jewish life" in today's Ukraine.

Not everyone agreed. One participant asked why Jewish scholars should even bother studying Ukraine, given its history of hostility toward Jews.

Ukrainian participants, meanwhile, noted that they were merely "invited to join" the conference, not asked to help organize or play a larger role — even though the talks were about their country.

Tyaglyy said the history of the Holocaust "is tied to the history of Ukraine, the controversial, amorphous history of this country that is still defining itself. That is why today's leaders are not touching this question. It is too difficult."