A 1919 painting by the German artist Otto Mueller at an exhibition at Berlin’s Jewish Museum. The 2008-2009 exhibition detailed the looting of Jewish art collections by the Nazi regime before and during World War II. (John MacDougal/AFP/Getty Images)

Since the discovery of a long-hidden trove of masterworks in Germany last year, advocates have sought to shine a spotlight on looted artworks hiding in plain sight.

In other words, those hanging on the walls of Europe’s great museums.

Enter France, known as the art attic of Europe before World War II and where tens of thousands of works were taken from Jewish families by the occupying Nazis. Today, more than 2,000 pieces returned to France after the war — including canvases by Claude Monet, Peter Paul Rubens and Max Ernst — remain in the custody of the Louvre, the Musée d’Orsay and other celebrated French institutions.

Under pressure for years to get such paintings to their rightful owners, French officials are now trying to clear the books on looted art. Instead of waiting for claimants to come forward, authorities are seeking to trace the ownership of some pieces as they belatedly try to unravel war-era mysteries.

But critics are decrying the French operation as too little, too late. At best, they say, museum officials are still not making restitution a priority, and at worst, they are dragging their feet in order to hold on to valuable works. The lumbering effort to find the owners of France’s orphaned art, critics contend, illustrates the lack of commitment by European governments to finally finish the job of returning property looted by the Nazis.

“What we get are fine words but not necessarily deeds,” said David Lewis, co-chairman of the Commission for Looted Art in Europe, a nonprofit group based in London that advocates on behalf of claimants. “Museum directors and curators begin to think of their collections as theirs and don’t wish them to be disturbed.”

A Sotheby’s employee early this year with a painting that the Nazis looted from a banker during World War II. (Emmanuel Dunand/AFP/Getty Images)

In 1998, 44 countries signed a landmark deal in Washington on assets looted by the Nazis, adopting a set of principles including the goal of identifying and returning looted art. Since then, however, the process has moved forward in fits and starts, with nations showing varying degrees of commitment.

In February, Germany announced the creation of a lost-art foundation with the aim of helping public museums and private institutes identify suspect pieces. Last year, Dutch officials concluded a four-year investigation into Nazi looted art in public collections, announcing the identification of 139 suspect pieces, including a majestic Matisse titled “Odalisque” in Amsterdam’s Stedelijk Museum.

Yet for each step forward, there have been complications and setbacks. Last year, for instance, a restitutions commission in the Netherlands ruled that three works in Dutch museums were products of a distress sale by Richard Semmel, a Jewish industrialist persecuted by the Nazis. But it agreed to return to his heirs only one minor painting. The commission refused to restitute two more-significant works, saying the rights of Semmel’s heirs carried “less weight” than those of the museums that currently owned them.

Here in France, tens of thousands of artworks were restored to their rightful owners in the years immediately after the war. But critics have blasted authorities for returning only 77 works — or less than 4 percent of the orphaned art collection — since 1951.

France’s Culture Ministry launched a new battle plan last year. A team of 15 experts was assembled to identify surviving heirs unaware of their rightful claims. The group, however, has worked largely part time, meeting roughly once a month. So far, officials say they may have identified the original owners of more than 20 works but have yet to discover the whereabouts of any of the owners or their surviving heirs.

Jean-Baptiste Pater’s “Le Musicien,” which was looted by the Nazis, was shown at Sotheby’s in New York in January. (Emmanuel Dunand/AFP/Getty Images)

Without a doubt, tracing ownership is no easy matter. Some of the works, said Marie-Christine Labourdette, director of museums for the French government, may simply have been returned to France in the 1940s because they were purchased here at one point, meaning that the original owners could have been from anywhere.

By actively trying to find the owners of orphaned works, Labourdette said, “France is clearly committing itself to repaying the damage done to Jewish families during the war.”

Yet some critics, such as Isabelle Attard, a French congresswoman, said the part-time program is simply not large enough to make a serious difference. She is demanding a full-time operation involving scholars from French universities and a host of investigators.

In addition, she and others are calling for a broader investigation of the general collections of French museums to identify other works that may bear the fingerprints of the Nazis.

French museums, Attard said, are part of the problem. She recounted a recent conversation in which the head of a major museum told her, “If we give back the paintings, the families will only sell them, so why return national patrimony?” She declined to publicly name the man.

French museum officials, however, insist that they have been nothing but supportive of the effort. If they have acted cautiously at times, they say, it is because they need to ensure due process in the restitution of important works.

Sotheby’s employees in January displayed a marking, “BoR 58,” made by Nazi looters on the back of Apollonio di Giovanni’s “Triumph of Marcus Furius Camillus, a Cassone Panel.” (Emmanuel Dunand/AFP/Getty Images)

Claire Chastanier, spokeswoman for the French Department of Museums, said the agency is willing to consider further reviews of art purchased by museums during the war years. But she largely dismissed calls for a broader review of France’s vast public collections, saying that would involve a “tremendous” effort to find what are likely to be relatively few suspect pieces.

“French museums are extremely cautious when it comes to their acquisition policy and pay special attention to determining the origin of the pieces of art that enter their collections,” she said.

Yet at least one recent example suggests otherwise.

Last year, the Musée d’Orsay purchased “Hercules Delivering Hesione,” by the German master Hans Thoma. Sold to the Nazis in the 1940s, the painting was destined for Adolf Hitler’s never-realized Führermuseum in Linz, Austria.

The work, however, had previously been sold under unknown circumstances to a gallery in Munich in 1939, and the d’Orsay opted to acquire it despite concerns about a possible taint raised by France’s Department of Museums, according to two people familiar with the purchase, who spoke on the condition of anonymity because of the sensitivity of the issue. Asked for comment, the museum’s press office supplied a statement saying that the work had been vetted by at least two investigations and that no evidence of despoliation or a forced sale during the Nazi era had been found.

But critics say the decision to buy such a painting suggests a lack of seriousness in dealing with looted art.

“We have made so little progress returning paintings, and they buy this?” said Corinne Bouchoux, a French senator who has advocated for more action on looted art. “What message does that send? The wrong one.”

Virgile Demoustier contributed to this report.