During his abbreviated lifetime, a cabaret performer named Fritz Grunbaum amassed a trove of artwork — more than 400 pieces, including 80 sketches and paintings by the Austrian expressionist Egon Schiele.

The works were ultimately plundered by the Nazis. For a while, many disappeared, until they began to resurface in auction houses and prominent museums.

On Wednesday, seven pieces were returned to Grunbaum’s heirs, who have been fighting for decades to reclaim the looted art.

The seven Schiele pieces, valued collectively at US$9.5 million ($16.05 million), were handed over to the family during a ceremony at the Manhattan District Attorney’s office, which has taken a leading role in tracking down stolen art and antiquities.

 “Your recovery of these artworks reminds us once again that history’s largest mass murder has long concealed history’s greatest robbery,” Timothy Reif, Grunbaum’s great-grandnephew and a federal judge in New York City, said of the state and federal authorities who made the handover possible.

By some estimates, the Nazis stole 650,000 works of art from 1933 to 1945, many from Jewish families who were arrested and then killed in concentration camps during the Holocaust. Some of the stolen pieces were created by some of the world’s most heralded artists, including van Gogh, Picasso and Chagall.

The modern style of many of the pieces was deemed “degenerate” by Adolf Hitler. He ordered some destroyed, while others he sold to help finance his invasions across Europe.

Artwork has been returned to the heirs of Fritz Grunbaum, a well-known cabaret performer who was arrested in 1938 and died in the Dachau concentration camp. Photo / AP

Grunbaum, who was also an actor and music writer, used the stage to throw barbs at the Nazi regime. Arrested in 1938, he was sent to the Dachau concentration camp, where he died three years later.

The return of the seven sketches and watercolour paintings follows a court victory in 2018 when a New York judge ruled that two works by Schiele had to be turned over to Grunbaum’s heirs under the Holocaust Expropriated Recovery Act, passed by Congress in 2016.

All of the seven pieces, like the two recovered earlier, will be auctioned to raise money to support scholarships for under-represented performing artists.

The nine works are a small fraction of the artwork being sought. The whereabouts of much of the collection remains unknown.

Two of the pieces returned today to Grunbaum’s family were voluntarily given back by New York City’s Museum of Modern Art.

Another, I Love Antitheses, which is valued at $2.5m ($4.2m) on its own, had been part of the Ronald Lauder Collection housed by the Neue Galerie in New York.

Two of the works were being held by the Vally Sabarsky Trust. A self-portrait of the artist was returned by the Morgan Library & Museum in New York, and a portrait of his wife, Edith, was given back by the Santa Barbara Museum of Art in California.

The DA’s office said the museums agreed to let go of the pieces “after they were presented with evidence that they were stolen by the Nazis”.

Last week, Manhattan authorities notified three museums of intent to seize three Schiele pieces, one each from the Art Institute of Chicago, the Carnegie Museums of Pittsburgh and the Allen Memorial Art Museum at Oberlin College in Ohio. Combined, the artworks are valued at nearly $4m ($6.7m). Those works will remain at the museums until they can be transported to the district attorney’s office, they said.

David Schaecter, president of the Holocaust Survivors Foundation USA, lauded Manhattan District Attorney Alvin Bragg for his efforts to find stolen artwork.

“Billions worth of looted assets remain in the wrong hands today, while families are forced to fight uphill battles against powerful institutions to recover their legacies,” Schaecter said.

The son of an Austrian Jewish art dealer, Grunbaum became a prolific art collector himself. While he was imprisoned at Dachau, he was forced to give his wife power of attorney, who then was forced to surrender the art collection to the Third Reich.

“The Nazis systematically murdered most of the Grunbaum family members,” Reif said.

“By recovering these long-lost artworks,” he said, “our law enforcement authorities have today achieved a measure of justice for the victims of murder and robbery.”