BERLIN (AP) — A German cultural institution has started investigating the provenance of about 1,000 human skulls from the country’s former African colonies that ended up in the collection of a Berlin hospital, officials said Thursday.

The collection of skulls, from colonies in what is today Rwanda, Tanzania, Burundi and Mozambique, were found in poor condition at Berlin’s Charite hospital and medical school and transferred to the Prussian Cultural Heritage Foundation in 2011.

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They are part of a bigger collection that was brought to Germany in the 19th and early 20th century by a German anthropologist, and experts have been cleaning and preserving the skulls over the last six years, the foundation said.

German researchers are now joining forces with an international network of scientists to review documents and find out more about the origins of the skulls, the foundation said. Among other things, they will be looking over inventories and correspondence from the time they were collected. They’re also doing non-invasive anthropological evaluation of the bones themselves to look for clues to their provenance.

If they can be properly identified, the plan is to return them to their countries of origin, though the foundation said no country has sought them.

“In order to return something, one needs to know where the specimens are from,” the foundation’s project leader Bernhard Heeb said in a statement.

The skulls were collected by anthropologist Felix von Luschan in the late 19th and early 20th century for research and were among several thousand taken from graves around the world, including South America and Europe.

After von Luschan’s death in 1924, the skulls were given to the Kaiser Wilhelm Anthropological Institute in Berlin. After World War II, they ended up being given to the Charite Hospital, for use in its medical history museum.

The hospital in 2010 decided it was not able to probably care for the collection, and turned it over to the foundation.

Though there were executions in colonial Africa, the foundation said as far as they can tell none of the skulls were those of such victims but were instead collected from graveyards.