Felix von Luschan probably didn't think that he was doing anything wrong. After the ambitious anthropologist took over the post of assistant director of Berlin's Museum of Ethnology in 1885, he gave the green light for a huge collection campaign: Europeans collected thousands of skulls and bones in various colonies and sent them to Berlin. Like other scientists of his time, Luschan wanted to use them to study human development. Many of the human relics ended up gathering dust in various storerooms for decades.

Some 5,500 human remains from Luschan's collections still exist. They're now owned by the Prussian Cultural Heritage Foundation, which runs a number of museums, archives and libraries in Germany.

"It's a difficult legacy," the foundation's director, Hermann Parzinger, told DW.

That's because the collectors often did not bother to ask for consent when they took the human remains. There was a real "collecting mania" in the African colonies at the time, Bernhard Heeb, an archeologist in charge of investigating the story of the skulls, told DW.

Hermann Parzinger wants to investigate the skulls' history

"In some cases, skulls were taken out of burial sites, sometimes in cloak-and-dagger operations," Heeb said. "Sometimes, human remains were also found lying in the open, but from today's point of view, it was obviously unacceptable to take them."

In addition to the official collectors, missionaries and colonial administrators also joined in and shipped various human remains to German research institutions. Colonial soldiers would occasionally alsosend the skulls of local people they had killed.

Descendants demand decent burials

Thousands of skulls and bones are still being kept in German museums up to this day – not just in the archives of the Prussian Cultural Heritage Foundation. The German public largely has no knowledge of this – unlike the descendants in Africa.

"For many people that I know, it's a terrible feeling that they have not been given the chance to bury their ancestors in a decent way," Mnyaka Sururu Mboro told DW. The Tanzanian-born activist is the co-founder of "Berlin Postkolonial," an association that aims to raise awareness about Germany's colonial past.

German museums ignored the dark remnants of the past that they had stored away in their archives for decades. "The crimes of the 20th century – World War I, World War II as well as the Holocaust – concealed the colonial past for long," Hermann Parzinger said.

Mnyaka Sururu Mboro says the skulls must be returned

But pressure has been mounting for the past few years. Berlin's Charite university hospital handed back several Herero skulls to Namibia between 2011 and 2014. Other organizations have also returned the human remains they had acquired.

The Prussian Cultural Heritage Foundation is about to follow suit. It started a project last year to investigate the exact origin and provenance of some 1,000 skulls in the Luschan collection. A Polish explorer had collected them in the former colony of German East Africa which encompassed today's Rwanda, Tanzania and Burundi.

Most of the skulls are believed to come from what is today Rwanda. "We do not have any inventories any more," Bernhard Heeb, who's running the project on behalf of the Prussian Cultural Heritage Foundation, told DW.

'Skulls shouldn't be returned to the basement'

Heeb's team is digging through archives, searching travelogues and other historic accounts for clues. In a second step, Heeb plans to travel to Africa and continue the investigations with African colleagues. The foundation says that it also plans to research the history of other skulls in its possession.

Some 1,000 skulls from the Luschan collection were taken from the former colony of German Southeast Africa

"We are doing research in order to return [the skulls]," foundation president Parzinger told DW. But there isn't any law in Germany that requires the foundation to do so. The German Association of Museums has only issued non-binding guidelines which recommend the return of all illegally-acquired objects.

The Prussian Cultural Heritage Foundation promises to hold talks with the governments of the countries the skulls were once taken from to discuss the next steps – including a possible return of the skulls.

"It's fine for us if the skulls are buried in their countries of origin," Parzinger said. "But they should not go back to the damp basements that we have just taken them out from."

African activists not satisfied

Activists like Mnyaka Sururu Mboro remain skeptical.

"It's always being said: 'We first need to investigate if the skulls came to Germany illegally.' If you use this approach, you can always find reasons to keep them here." Mboro insists that it's not a problem to investigate the origin of the skulls. "There are plenty of people in Tanzania who know where a person was beheaded and his skull was taken to Germany."

Thousands of skulls could still be stored in German archives

Some graves have even been marked accordingly, Tanzanian historian Reginald Kirey told DW. "The family of Chief Songea Mbano designed the grave in a way that it indicates that the skull is missing. It has two partitions: The first partition shows that the skull is not there, the other parts of the body are in the second partition," Kirey, a lecturer at the University of Dar Es Salaam, now studying for his doctorate at the University of Hamburg, said. According to media reports, Tanzania's government is considering demanding the return of Songea Mbano, a famous chief who was executed by German colonial troops in 1906.

Kirey says that the African communities need to be involved in the process of investigating the origin of skulls.

"You need to listen to these people and find out what their demands are. They should give the direction of how to start searching and returning the skulls. It has to adhere to their customs."

Nobody expects the debate about the skulls to end soon - and many observers expect it to widen.

"Such collections do not just exist in Berlin, but in Paris and London as well. It is going to be a topic there as well at some stage," the Prussian Culture Heritage Foundation's Hermann Parzinger said.