In July of last year, black bee hives were introduced to Wisbech Castle in England, as part of an effort to conserve the rare critters. Now, thousands of the castle’s bees are feared dead, following an inexplicable attack by two intruders.

According to the BBC, CCTV footage from the early morning hours of January 8 shows the suspects breaking onto the grounds of Wisbech Castle, which is believed to have first been built by William the Conqueror in the 11th century; the structure that stands today dates back to the late 1700s. The intruders lifted the lids off the hives, kicked them, and then attacked the bees inside with sticks.

Though they aren’t rendered completely dormant by cold weather, honey bees enter into a reduced state of activity once winter hits, forming clusters inside their hives to stay warm. The buzzing insects would have been “helpless to defend themselves” from the attack, writes Colin Drury of the Independent. Castle staff members aren’t entirely sure of the extent of the damage—they won’t be able to check properly until March, because further exposure to cold air could kill any surviving bees—but they estimate as many as 10,000 bees might have died.

Kirsty Hulley of the Cambridgeshire police called the incident “a cruel, unprovoked and completely unnecessary act of violence.” Steve Tierney, a local councillor and chairman of the Wisbech Castle committee, was less diplomatic, calling the intruders “morons,” according to the Wisbech Standard.

“There was no reason to do this, there was nothing to be gained,” Tierney added. “[I]t was simply to destroy bee hives and bees for some sort of sick pleasure of destruction.”

The attack has been rendered all the more devastating by the rarity of the British black bee, also known as the dark European honey bee, or Apis mellifera mellifera. This subspecies, which is native to Britain, was thought to have all but died out until several colonies were identified in 2012.

The bees’ range remains greatly reduced “as a result of importation and replacement of queens with those of other Apis subspecies,” according to a 2018 study in the Journal of Apicultural Research. A. m. mellifera populations have been “heavily hybridized,” the study noted, and conservations say that preserving pure colonies is important for the sake of biological diversity. In the U.K., both wild bee species and managed honey bee hives have been declining in recent decades, due to factors like pesticide use, habitat loss and disease.

It is expected to cost Wisbech Castle £2000 (around $2,640) to restart its conservation project in the spring. Police have asked anyone with information to come forward.