Thieves stole 21 ancient human skulls from the basement of a church in Kent, U.K.

Twenty-one ancient human skulls have been stolen from the basement of a church in the British county of Kent.

St. Leonard’s Church, located in the town of Hythe, is renowned for its massive collection of ancient skulls, many of which are put up on display in the church’s ossuary, or bone chamber.

But earlier this week, thieves armed with a bolt-cutter broke into the ossuary and swindled 21 skulls from the display, Kent Online reported. The skulls are believed to be around 700 years old.

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Rev. Andrew Sweeney, the priest-in-charge at the church, said the crime appeared to have been committed by professional thieves rather than mischievous children or teenagers.

“These people came with the right equipment to break the gate,” Sweeney told Kent Online. “We believe they came specifically for the skulls. They didn’t take money from inside, and there was no vandalism. They just took the human skulls and left.”

Sweeney said he was concerned the act of skulduggery may have been motivated by a desire to sell the remains on the black market.

“We have always assumed human decency in our visitors and compassion for those whose remains rest in peace in the sacred space of our church. We are saddened that the greed, selfishness or stupidity of some people has destroyed that assumption of common human values,” Sweeney wrote in a blog post. “We have now had to resort to expensive and complex security measures which we once thought unnecessary.”

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Authorities are asking the public to come forward with any information on the macabre theft.

“We recognize this is perhaps an unusual theft, but these skulls were not free for the taking,” said Inspector Maxine Harris. “They are part of an important collection, and we are keen to see them back in their rightful place in the crypt.”

The ossuary contained some 1,022 skulls prior to Monday’s theft, according to the St. Leonard’s Church’s website.

The skulls have been the subject of several forensic and anthropological studies, with researchers concluding that they belong to Hythe residents from different walks of life rather than predominantly war dead or slain Danish pirates, as has previously been theorized.

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Many of the skulls were dug up from area graveyards in the 15th century, possibly to make room for the hordes of people who died during the Great Plague, according to the BBC.

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