An unknown predator has bitten two penguins to death at a zoo in Emmen in Drenthe in what officials say is a rare attack.

The owners of the Wildlands zoo said the animals must have been killed by an outside predator, such as a fox or marten. No other animals had escaped from their enclosures, spokeswoman Hanneke Wijshake told broadcaster RTV Drenthe.

“The penguins had bite marks, but we’re not sure from which animal. A marten or fox may have done this or perhaps a feral cat or polecat,” zoo worker Henk Kuiper said.

The zoo is still investigating where the animals might have entered the penguin enclosure, which has now been cordoned off with electric wire to protect the remaining penguins. A wildlife camera has been installed to detect the culprit.

Such attacks are rare, Kuiper said. In 2003, 11 penguins at Burgers Zoo in Arnhem were bitten to death by a fox.