(CNN)Nina McGehee always knew that her family fled Europe before World War II broke out. But she only recently learned about what they left behind.
A German auction house is helping return Nazi-looted art to their rightful owners
McGehee's great-grandfather, Siegfried Laemmle, was a German art and antiquities dealer with a successful business in Munich. He was also Jewish, and like all Jewish art dealers in the Third Reich, he was forced to liquidate his business as part of the "Aryanization" of businesses.
Some Laemmle objects ended up in private collections, others in museums. Most were sold through a Munich auction house under the guise of legitimate sales.
Now, the current head of the auction house is helping return those objects to their rightful owners, including McGehee's family.
A common estimate for the number of works of art looted in the Nazi era is 600,000 with at least 100,000 still missing.
Often, in the process of restitution, the burden is on families to prove an object was sold because of Nazi persecution. That's starting to change as countries and institutions strengthen their commitments to researching the provenance of objects in their possession and making that information publicly available.
A central database contains an estimated 25,000 missing, looted or identified objects -- paintings, drawings, antiquities -- from more than 15 countries.
But Neumeister Auctions in Munich appears to be one of few private art dealers in the world taking the work upon themselves.
"I'm not responsible for what happened in National Socialism (Nazism), but I'm responsible for what I'm doing now -- or especially what I'm not doing," said managing director Katrin Stoll, who took over the business in 2008.
"As a German business, we have a social responsibility to do this."
Stoll and McGehee shared their story this week at the University of Denver's Center for Art Collection Ethics, where McGehee recently audited a class on provenance research. Toward the end of the semester, McGehee revealed to the professor her connection to the subject.
"It's a wonderful success story that shows the benefits of the kind of research that we're trying to promote," said associate professor Elizabeth Campbell.
"It's an excellent example of how provenance research can help families achieve belated justice as well as strengthen human connections within one family and across continents."
McGehee says the restitution process opened her eyes to a chapter in her family's history that her parents seldom spoke of, eager as they were to leave it in the past.
"I didn't know anything about the family business or that my great grandfather was considered one of the top-of-the-line dealers," McGehee said in a phone interview. "It just wasn't something my parents ever discussed."
McGehee's journey to the truth began with a German documentary that includes her family's story.
"Under the Hammer of the Nazis" recounts Stoll's efforts to right the wrongs of a key figure in the trafficking of Nazi-plundered goods in Bavaria. The documentary was screened at the University of Denver event that included McGehee, Stoll and the researcher who reviewed Neumeister's files, Meike Hopp.
Art dealer Adolf Weinmueller was a Nazi party member who directed the pilfering and sale of the Laemmle collection, among others in Bavaria, for the Reich Chamber of Fine Arts, which oversaw the liquidation of the Jewish art trade.
Adolf Hitler intended for the artifacts to end up in the Führermuseum, an unrealized art museum in a planned cultural complex for his hometown. Hitler had first pick of the looted objects, according to the film, but most of them went into private ownership through auction.
Siegfried Laemmle was 73 when the order came to liquidate his collection. A respected dealer and collector, he had been running his business in Munich since 1894, specializing in late medieval and early Renaissance sculptures.
Laemmle and his son Walter tried to convince authorities of their professional qualifications and commitment to German culture. But they quickly realized they had no choice but to dissolve the business.
Laemmle sold off as much as he could before immigrating with his family to California, where his brother, Carl Laemmle, the founder of Universal Studios, lived. The rest eventually fell into Weinmueller's hands as the person overseeing the liquidation of the region's Jewish art trade.
With the Laemmles and other Jewish competitors gone, Weinmueller's business flourished as he auctioned off their plundered objects.
Just how many objects passed through his auction house, or where they went, was a mystery until the past decade, when a discovery in the auction house basement shed light on the inner workings of Weinmueller's business.
For years after the war's end in 1945, historians knew Weinmueller was an agent of the Nazi party. But without records, they had no way of enumerating the nature of his misdeeds, let alone prove them. He was allowed to keep his business until his death in 1958, when German businessman Rudolf Neumeister took over the business.
In 2008, Neumeister's daughter, Stoll, took over. Knowing what she did about Weinmueller's reputation, Stoll decided to give the business a fresh start under her tenure.
"I couldn't explain what I was searching for, I just had a feeling there was a history with Weinmueller, but nobody could give me answers," she said.
Ten years earlier, 44 countries committed to the Washington Principles on Nazi-Confiscated Art, a set of principles that charged participating nations with returning Nazi-confiscated art to their rightful owners. But the principles were nonbinding, and no formal framework was created to help governments or private entities take up the task.
Stoll hired provenance researcher Meike Hopp to create a system to investigate the sales.
During her research, the pair made an unexpected discovery in the basement of the auction house: bundles of auction catalogs filled with notes in Weinmueller's handwriting.
The level of detail in the catalogs was like nothing that had been discovered before, Hopp said. The catalogs included the names of Jewish art dealers or businesses, descriptions of their objects and the buyers.
In one auction alone, according to Weinmueller's catalog, 159 hand drawings and graphics that once belonged to Laemmle were sold.
The pair digitized the annotated catalogs and published them in 2014 on Lost Art, a database of Nazi-looted objects. Finally, the nature of Weinmueller's misdeeds was laid bare.
"He was one of the main figures to catalyze the process and support the Nazis in its activities against Jewish art dealers," Hopp said. And he was one of the main profiteers, she said, because he received the private collections and belongings Jews were forced to leave behind when they fled the country.
McGehee says she knew that her family had artifacts in Europe. Her parents, who lived in Minneapolis, first got involved in restitution efforts in the early 2000s, while she was in Denver, tending to her own family and career.
Her great-grandfather died before she was