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Heidi Sieker, 44, is the German-born daughter of an unrepentant Nazi. Sonia Gumpert, 79, is the German-born daughter of a Jewish factory owner. In another time, they might have been neighbors. In another time, they might have been enemies.

Instead, the Nazi’s daughter and the Jewish immigrant are the unlikeliest of friends.

When the two sit together in Gumpert’s Marin County home, chatting away in German, there’s no trace of the nightmares of the past. Sieker recently flew to America to visit her friend, who’s been living in the Bay Area for decades. It was purely a social call.

The two drove down the coast to visit Gumpert’s cousin in Carmel. By now Sieker has gotten to know many members of Gumpert’s far-flung family. In fact, she put together an extensive family tree, one that includes 230 individuals stretching back to 1770.

Why would a young German woman, educated in Catholic schools and admittedly ignorant about the Holocaust until she went to college, take such a strong interest in this one Jewish family?

It gets down to brass tacks. Literally.

Gumpert’s father — like his father and grandfather before him, owned the Messenhof brass factory in the central German town of Kassel. Gumpert spent her first years in Kassel, but once Hitler came to power in the early 1930s, her father invoked his Swiss citizenship and took the family to Basel, Switzerland, where they rode out the war years.

Though Swiss Jews were safe from Nazi persecution, life was not easy for them. Gumpert’s father lost his factory and faced many obstacles launching a business in Basel. Facing the stress of exile, a frivolous lawsuit and other harassment, he died of a sudden heart attack before the end of the war.

“His business killed him,” recalls Gumpert. “We came to California right after the war because my mother thought we would have more opportunities there.” She was right.

Gumpert married an American man, worked as a nurse and raised a family, gradually forgetting the pain of the war years. “The factory,” she says, “was in the past.”

Not so for Sieker. Having majored in art and communications at the University of Kassel, she became a filmmaker. When the city decided to convert the abandoned Messinghof factory into a cultural center in the mid-1990s, Sieker got the idea to make a film about its history.

Up until then, she had learned very little about the Holocaust. Growing up, she heard mostly how the Americans had mistreated her father in a POW camp. For him, the war never ended, and neither did the ideals of the Third Reich.

But the daughter wasn’t content to accept his view. After college, Sieker traveled to Israel to live on a kibbutz. She stayed nearly four months.

“I thought it was like the world coming together,” she says. “It impressed me a lot.”

So when she learned the brass factory had been owned and operated by three generations of Jews, she became even more drawn to her subject. “The family owned it from the 1860s until Hitler,” she says. “That’s when the factory was stolen.”

To make her film come alive, Sieker tracked down as many members of the Lieberg family she could find. (Lieberg is Gumpert’s maiden name).That led to a meeting between her and Gumpert in Basel in 1998.

“I thought it was great,” recalls Gumpert of her first meeting with the blond, blue-eyed filmmaker. “Someone was interested in my family.”

Sieker went on to locate other Lieberg family members in America, Australia and Britain, including a few who were part of the famed Kindertransport. But during production of the documentary, she grew especially close to Gumpert.

In 1998, Sieker completed “Pieces of Memories,” which was distributed throughout Germany. She went on to make other films, including a 2001 documentary on the Kindertransport, titled “Flight of the Children.”

As satisfying as her work has been, Sieker admits one person in her life still doesn’t understand all of her outreach to Jews: her father.

“He wants me to do a story about him,” she says. “He feels like a victim. He came from a poor family, was uneducated and was only 17 when he went into the Luftwaffe. Before the war, he only met one Jew in his life. It was in his class at school. It was a Lieberg.”

But she adds: “He says I have done more for the Jews than some political leaders in Germany. He just has to accept that he followed the wrong ideas.”