PARIS — With its monumental Art Deco facade overlooking the Eiffel Tower, the Musée de l’Homme, or Museum of Mankind, is a Paris landmark. Every year, hundreds of thousands of visitors flock to this anthropology museum to experience its prehistoric skeletons and ancient statuettes.

But beneath the galleries, hidden in the basement, lies a more contentious collection: 18,000 skulls that include the remains of African tribal chiefs, Cambodian rebels and Indigenous people from Oceania. Many were collected in France’s former colonies, and the hoard also includes the skulls of more than 200 Native Americans, including from the Sioux and Navajo tribes.

The remains, kept in cardboard boxes stored in metal racks, form one the world’s largest human skull collections, spanning centuries and covering every corner of the earth.

But they are also stark reminders of a sensitive past and, as such, have been shrouded in secrecy. Information on the skulls’ identities and the context of their collection, which could open the door to restitution claims, has never been made public, but is outlined in museum documents obtained by The New York Times.

A confidential memo said that the collection included the bones of Mamadou Lamine, a 19th-century West African Muslim leader who led a rebellion against French colonial troops; a family of Canadian Inuits exhibited in a Paris human zoo in 1881; and even five victims of the Armenian genocide in the mid-1910s.

“Sometimes, the supervisors would say, ‘We must hide,’” said Philippe Mennecier, a retired linguist and curator who worked for four decades at the Museum of Mankind. “The museum is afraid of scandal.”

That opacity has been at odds with France’s growing reckoning with its colonial legacy, which has shaken many of its cultural institutions. It has also hindered claims for restitution of items from former colonies or conquered peoples, in which human remains are often named as a priority — an issue currently roiling Europe’s grand museums.

While France has led the way in Europe in investigating and returning colonial-era collections of artifacts — cultural objects, made by human hands — it has lagged behind its neighbors when it comes to remains.

Museums in Germany, the Netherlands and Belgium have all developed clear protocols for dealing with remains, with different restitution criteria from artifacts. Claims for cultural objects usually consider the conditions under which they were taken; for remains, a claimant usually just has to prove an ancestral connection. In several recent high-profile cases, museums in those countries have returned skulls and mummified heads, with promises for further transparency and accountability.

In the United States, a 1990 federal law has facilitated the return of Native American remains, although restitutions have moved at a slow pace. A number of prominent universities and museums, including University of Pennsylvania Museum of Archaeology and Anthropology and the Smithsonian Institution have discussed, and in some cases, developed policies for how to deal with the remains of enslaved people that are held in their collections.

But in France, critics say, the Museum of Mankind limits research into sensitive items in its collection, withholding essential information for restitution claims. The museum has a longstanding policy of only returning “nominally identified” remains, meaning corpse fragments from a specific person with a connection to the claimant. Some scholars say it is a restrictive tactic designed to block returns.

Christine Lefèvre, a top official at the Museum of Natural History, which oversees the Museum of Mankind, said, “The collections are open to anyone who comes with a solid and serious research project.”

What is more, French legislation has made any return a cumbersome and time-consuming process.

“Our museums should do some soul-searching,” said André Delpuech, a former director of the Museum of Mankind who left that post in January. “But so far, it’s been a head-in-the-sand approach.”

As with other 19th-century museums, the museum was initially a repository for items gathered from around the world. The skulls were collected during archaeological digs and colonial campaigns, sometimes by soldiers who beheaded resistance fighters. Prized by researchers working in the now-debunked field of race science, the remains then fell into relative oblivion.

In 1989, Mennecier, the curator, put together the first electronic database of the collection. It enabled him to identify hundreds of what he called “potentially litigious” skulls — remains of anticolonial fighters and Indigenous people, collected as war trophies or plundered by explorers — that could be claimed by people wishing to honor their ancestors.

Sensing potential trouble as restitution claims increased internationally, Mennecier said he warned museum leaders multiple times over several years about the sensitive remains, urging them “to inform the highest government authorities, possibly the embassies, the relevant communities.”

But those calls went unheeded, he and Alain Froment, an anthropologist at the museum said, leaving foreign governments and Indigenous communities in the dark.

“It’s incredibly difficult to understand what’s in their collection,” said Shannon O’Loughlin, the chief executive of the Association on American Indian Affairs, a nonprofit promoting Native American cultural heritage. She added that her “heart fell” when she learned of the Sioux and Navajo skulls in the Museum of Mankind basement.

The museum has published only a stripped-down online version of its skull database, sharing no names or biographical details, even though the list seen by The Times contains this information about hundreds of remains.

Lefèvre and Martin Friess, who is responsible for the Museum of Mankind’s modern anthropology collections, said the information was withheld because of privacy concerns, fear of controversy and because of uncertainties around some remains’ identities.

For instance, the provenance of a skull listed as belonging to a Sioux chief named White Cloud was in doubt, said Friess, who has researched the case further.

But several scholars and lawmakers said the museum’s stance stemmed from a greater concern: that transparency could open the floodgates for restitution claims.

Like other institutions, the Museum of Mankind has faced growing repatriation requests — from countries including Madagascar and Argentina, and from Indigenous people in Hawaii. But unlike many counterparts in Europe and the United States, the museum has not invested significantly in provenance research for its human remains collection, nor published guidelines for their handling and return.

Over the past two decades, France has returned only about 50 sets of remains, including to South Africa, New Zealand and Algeria. By comparison, Germany returned eight times as many over the same period, according to a researcher at Brandenburg Medical School.

“It does make France seem behind,” said Jeremiah Garsha, a historian at University College Dublin, noting that the country “has a much longer colonial history and less of a track record” than Germany.

Part of the reason for this discrepancy are policies like the Museum of Mankind’s nominal identification requirement. Plans to return Australian Indigenous remains in the collection, most of which are unidentifiable, have stalled as a result, according to Mennecier and Froment.

That policy, however, is not shared by other European museums and “has no clear legal basis,” as noted in the confidential museum memo. It also contradicts a 2018 government-commissioned report, also obtained by The Times, which recommended considering as returnable anonymous remains that could be connected to a family or an Indigenous group. (The report, which encouraged France to take a proactive stance on restitution, was never made public and its proposals were never enacted.)

Lefèvre, the museum official, said that community affiliation was too vague a criterion, noting that connections with 19th-century groups were hard to establish. But she added that anonymous skulls of individuals whose social functions can be determined, such as tribal leaders, could be deemed returnable.

Klara Boyer-Rossol, a historian who has studied remains from Madagascar, said the museum’s identification policy was restrictive, unrealistic and possibly designed to limit restitutions.

“It’s completely hypocritical,” Boyer-Rossol said, adding that most of the skulls were collected without documentation and that, in her view, the museum puts up obstacles to academic research despite recent efforts at transparency. It took her 10 years to gain full access to the museum’s database on Madagascar, she said.

To make matters more complicated, objects in public museum collections are the property of the French state and cannot change ownership unless the return is voted into law — a cumbersome process that has sometimes led France to lend remains instead of ceding possession.

A representative for France’s culture ministry said officials were working on a sweeping law to regulate future returns of human remains.

But Pierre Ouzoulias, a left-wing French senator who has produced several reports on restitution, said the government had shown anything but good will. It has rejected a Senate proposal to establish a scientific advisory council on restitutions and has yet to examine a