A Swiss museum director preparing for a Naziera art collection's long-awaited public unveiling later this year said Friday that her goal remains finding heirs to any works that may have been looted from Jewish owners.

Bern Museum of Fine Arts head Nina Zimmer, who took ownership of 150 drawings, lithographs and paintings this week ahead of an exhibition slated to begin in November, said research shows none of these were stolen by National Socialists.

But questions linger over the provenance of some of the collection's pieces still in Germany, where a 2012 raid by authorities on a Munich apartment produced a sensation: 1,500 long-lost works by modern masters, including Pablo Picasso, Otto Dix and Henri Matisse.

"Every restitution is a victory for us," Zimmer said in an interview, while acknowledging such provenance sleuthing remains unpredictable. "I cannot make any promises."

Additionally, the Bundeskunsthalle in Bonn, Germany, is also planning to display items from the collection, which to date has produced only five works confirmed to have been stolen by the Nazis.

Four have been returned to heirs, so far, including a Matisse portrait, Sitting Woman, that belonged to Paris-based collector Paul Rosenberg.

Before its discovery five years ago, the massive trove was hidden for years in the German and Austrian homes of Cornelius Gurlitt.

His art-dealer father, Hildebrand, amassed it after being enlisted by the Nazis to sell so-called "degenerate" modern art they had seized from German museums.

Though original estimates for the collection's value topping $1 billion were likely exaggerated, experts said, the find is still spectacular.