Lt. Alexandre Rosenberg of the Free French forces pounded on the boxcars. Hold your fire, he told his men. There might be prisoners inside the train they had stopped outside Paris in August 1944. Slowly, a few of the heavy doors slid open, and a handful of worn German soldiers straggled out.

Inside, the French found the cargo the Germans had been guarding, crates jammed with artwork: sculptures, drawings and framed paintings, some stacked against one another like bread slices, their signatures visible.

Picasso. Renoir. Braque. Cézanne.

The lieutenant did not need to read the names. He had seen many of the works before, hanging in the Paris home of his father, Paul Rosenberg, then one of the world’s leading dealers in Modern art.

Since that day, three generations of Rosenbergs have been engaged in a painstaking search for hundreds of artworks that were looted from their family by the Nazis. This month their hunt led to Norway, where the family is negotiating for the return of a Matisse that has hung for 45 years in the Henie Onstad Arts Center, a museum founded by the skater Sonja Henie and her husband.