VIENNA—An Austrian natural-history museum filled with artifacts once collected by a Nazi unit akin to the one portrayed in "Raiders of the Lost Ark" has begun returning looted mammoth bones, stuffed birds and hunting trophies to their proper owners.
The restitution push by Salzburg's Haus der Natur follows a three-year research project into the provenance of its collection. The museum is now examining thousands of books and objects confiscated from religious organizations, private citizens and institutions across Europe before and during World War II.
The high-profile restitution drive comes as much of the spotlight has been trained on similar research efforts within the fine art world—most notably that behind a large trove of works found in the Munich apartment of a former Nazi dealer's son, which German authorities disclosed late last year.
The Salzburg museum, founded in 1924 by Eduard Paul Tratz, became part of a Nazi archaeological organization known as Ahnenerbe, meaning ancestral heritage. Mr. Tratz, a member of the Nazi party, is widely known to have used the arrangement to acquire looted objects for the museum.
"He was a very active opportunist who used all possibilities to support his museum," said Robert Hoffmann, professor emeritus of history at the University of Salzburg, who was asked by the museum to help reappraise its past.
Haus der Natur returned many plundered artifacts soon after the war, but hundreds of others remained in its possession for decades. Under new leadership, the museum in 2010 began researching its role during the Nazi regime, in particular that of Mr. Tratz, spokeswoman Charlotte Kraus said.
But, the museum added, "from today's perspective it undoubtedly would have been better to begin a historic and scientific evaluation" in the 1990s, when Mr. Tratz's activities first drew more widespread attention.
Austria has a relatively good track record on the restitution of stolen artwork and objects in public collections, said Oliver Rathkolb, a professor of contemporary history at the University of Vienna. The country's 1998 art-restitution law was updated five years ago to make it more accommodating to heirs with possible restitution claims, and commissions now actively contact them, he said.
But Austria still faces challenges tracing once-looted pieces now in private hands. In these cases, lawful owners must rely on the goodwill of the private institutions, or hope that pressure from the public or the market will sway them. Galleries and auction houses often refuse to conduct business with collectors known to own art of looted or questionable provenance.
Mr. Tratz was imprisoned by American forces after the war but released when the prison closed. He returned to head the museum in 1949 and served as director until 1976, a year before he died. He remains a contentious figure. In 1963, the city of Salzburg awarded him honorary citizenship, a gesture that a 2007 petition unsuccessfully sought to overturn.
"As long as he lived, he was highly regarded," and socialized with Salzburg high society, Mr. Hoffmann said. Willingness to overlook Mr. Tratz's Nazi past was typical of the era, he said, and close ties to the museum eased his return there.
Discussions about Mr. Tratz's legacy began after his death, Mr. Rathkolb said. But the size of the collection—around 900,000 objects—made research into its origins difficult, he said.
Robert Lindner, head of collections at the museum, said its unwieldy storage area and the need for extensive archival cross-checking also complicated cataloging.
"We would like to finish all the talks [with the rightful heirs] and return everything this year," Mr. Lindner said. "However I can't exclude the possibility that in five years or so that new cases will show up."
The Ahnenerbe was controlled by Heinrich Himmler, the head of the elite SS police force. The group launched expeditions and excavations throughout the territories occupied by the Nazis and farther east trying to prove Germanic racial superiority and find evidence of Aryan prehistory.
Along the way, it plundered museums, institutions and universities, as well as the collections of individuals, as portrayed in the 1981 Steven Spielberg movie. Himmler visited Haus der Natur in 1938, and Mr. Tratz joined both the SS and the Ahnenerbe. In the post-war years, Mr. Tratz defended his SS membership as protecting and augmenting his collection.
In Poland, one target was the Warsaw Zoological Museum, from which the museum says Mr. Tratz took objects, including a stuffed water buffalo, hundreds of birds and microscopes. He also participated in looting in the former Soviet Union and France.
Haus der Natur still holds a bird collection it believes came from the Warsaw museum and has invited museum representatives to Salzburg to claim them.
The property of Jewish citizens also landed in the museum's hands. Art and artifacts collected by Alphonse and Clarice Rothschild, members of the banking family, were confiscated by the Nazis in 1938, and the museum says Mr. Tratz arranged to have their African hunting trophies transferred to the museum in 1940.
The museum was in contact with the Rothschild family after the war, but it is unclear whether any objects were restored, it said. Haus der Natur is now examining the origin of more than 100 African hunting trophies, including antelopes, zebras, gazelles, lion skins and an Egyptian vulture, and is in touch with the family again.
The provenance of some objects remains unclear, including an assortment of South American birds marked "B. Boni-Quito, Ecuador." After contacting several museums in Europe, objects with similar markings were found in a natural-history museum in Paris. The museum is investigating a possible connection to the journalist Georg Gregor Bomstein-Boni, who was deported from France to the Sobibor and Majdanek extermination camps, where he was killed in 1943.