Yaki Gantz’s mission began nearly a decade ago with a tombstone covered in numbers.

Gantz, a former Israeli secret service agent, was attempting to reinvent himself as a tour guide in Książenice, Poland in 2004 when he came across an odd mass grave marked with 45 sets of numbers, reports the Times of Israel. Intrigued, Gantz did some investigating and learned the grave belonged to Holocaust victims killed in 1945.

The Rehovot, Israel native has since made over 100 trips to Poland, partnering with Yad Vashem, Israel’s official Holocaust memorial in Jerusalem, in an effort to identify dozens of unmarked or anonymous Holocaust grave sites across the country.

“In every journey you discover more and more new details and the question of how it all happened only intensifies,” Gantz told the Times of Israel.

An important part of Yad Vashem’s mandate is to track down similar sites and piece together what happened: who was killed, when, and how. Naming the victims is important, according to Yad Vashem, as it is a name “which turns a person from an anonymous statistic into a flesh-and-blood human being.”

Ultimately, Gantz and Yad Vashem were able to identity 24 victims murdered and buried near Książenice, Poland, just some of the thousands forced into a brutal death march from Auschwitz-Birkenau in January 1945. Thanks to these efforts, David Pastel, Samuel Kops and their fellow prisoners are no longer mere numbers on a stone slab.

Knowing the exact location of a relative’s grave is a priceless gift for some families who had previously believed their family members lost forever to Auschwitz’s ovens.

Amos Cohen, a Haifa shipbuilder, was similarly given this closure with the help of Gantz, who was part of a team that identified the grave of Cohen’s long-lost relative Rose Kobylinski, reports NBC News.

Kobylinski too died in a German death march, but was ultimately buried alongside nine other prisoners in the cemetery of St. Anna’s Roman Catholic Church in Swierklany, Poland.

“Their relatives now know that their relatives didn’t just become ashes at Auschwitz,” Gantz told NBC News. “They know there is a place where they can come to say Kaddish.”

The tour guide also helped unlock the secrets of yet another mass grave of victims from the Auschwitz death march, working to identify the 42 victims buried the Polish town of Miedzna, reports Haaretz.

For his part, Gantz said he will not rest until as many Polish Holocaust graves as possible are unearthed and properly identified. In part, this is a way to thank the Polish citizens who in many cases retrieved Jewish victims murdered by Nazis and made sure they at least were given a proper burial, according to the Times of Israel.

“It is amazing to see how many people helped the Jews then, and how many people want to help me now,” Gantz told the site.