Genya Buslovich recalls the somber anniversaries marked by the Jewish families in her childhood home in Lithuania in the 1950s. Each year, these Holocaust survivors would board an old bus, travel to the outskirts of a small town, walk quietly to a ravine — and pray.

The ravine was one of thousands of sites across Eastern Europe and Russia where Jews were rounded up in their own towns during World War II and killed, in a genocide strategy far less known than the concentration camps.

On Thursday, Buslovich met the world's top investigator of those massacres — the Rev. Patrick Desbois, who spoke to a crowd of about 800 at Beth El Synagogue in St. Louis Park as part of International Holocaust Remembrance Day. He offered to help her learn more about such sites during her visit to Lithuania later this year.

"I remembered people stood there and cried," said Buslovich, of Plymouth, of her childhood memory. "My parents went every year."

Desbois is a French Catholic priest who has spent the past 15 years on a mission to reveal this hidden side of the Holocaust, and to ensure that the estimated 1.5 million Jews murdered that way are not forgotten.

Featured recently on "60 Minutes," Desbois, a professor at Georgetown University in Washington, D.C., has interviewed more than 5,000 eyewitnesses about 2,100 execution sites across the former Soviet Union.

Many had never talked about what they saw happen to their neighbors, shopkeepers, classmates and others who were rounded up, massacred and dumped into mass graves.

While stories of such atrocities were known, the details of what happened and where had been unspoken, said Desbois.

"Most people thought it was a smaller group," said Desbois, president of Yahad-In Unum, a Paris-based organization dedicated to documenting these unmarked massacre sites.

"And the killers could be anyone — soldiers, a farmer, a woman," he added.

Desbois explained how he and his team would enter a village and search for witnesses to atrocities committed 70 years ago. Many described a common pattern in the Soviet nations. The night before the massacre, the Jewish quarter would be surrounded by local police. Girls were raped, valuables stolen. The next day the Jews — often told they were being deported — would be marched ostensibly to the train station. But a mass grave would be dug nearby, and they be systematically slaughtered.

Their homes were then raided, the contents stolen or auctioned off. All signs of Jewish life — gone. It was a scene repeated thousands of times, across thousands of miles.

Many mass graves

The grave site prayers witnessed by Buslovich as a child showed how common the massacres were. She recalled going to different places with her family, depending on the anniversary dates of the killings being marked.

The mass graves weren't just in ravines and woods. Desbois has found them in farm fields, under houses, even under a woman's tomato garden.

"I tell them, 'Finally, we've found you,' " he said.

Curious about Desbois' investigations, Buslovich visited the website of Yahad-In Unum, which has an interactive map of massacre sites. A dot appears next to the city of Vilnius, where she grew up. Readers can click on it and learn of three massacre sites in the region, and view eyewitness accounts.

The parents of her husband, Joseph Buslovich, were from the towns of Berezino and Mogilev in Belarus, which are also on the map. He, too, is eager to learn more.

How many other Minnesota Jewish families can trace their family members back to these unmarked graves is unknown. But for many, it will be the first chance to get details of what happened to them.

Buslovich, who grew up in what was then Leningrad after his parents left Belarus, said everyone knew what had happened to the vanished Jews, but nobody would talk about it.

"It was a hidden subject," he said. "They didn't even mark the places."

Desbois stressed that genocide is not a thing of the past, saying it is happening right now to the Yazidi people, a religious and ethnic minority being systematically wiped out by ISIL militants in Iraq.

He described genocide as a slaughter in which killers never think, 'This woman is too old. That baby is too young." Everyone is killed.

Desbois' visit was sponsored by the Jewish Community Relations Council of Minnesota and the Dakotas.

"People knew the existence of the German brigades who machine-gunned hundreds of thousands of people," said Steve Hunegs, the council's executive director. "Until this research, there was not much known how it came to pass. … [Desbois] has transformed abstraction into reality."