Animal advocates are offering a $5,000 reward for information about the killing of at least three platypuses in Australia, two of which were beheaded.

The bodies of the protected duck-billed mammals turned up in the Albury Botanic Gardens in New South Wales over the last five weeks. Tests show that other animals did not cause their deaths. The beheadings, for example, were carried out by someone who used a sharp object.

The Humane Society International Australia is offering the reward in hopes of finding out who’s responsible for the “despicable act of cruelty,” the organization stated. “These crimes are truly shocking towards such gentle and defenseless animals, and one of our beloved Australian icons.”

“It’s just sad,” Hazel Cook of the Wildlife Information Rescue and Education Service told The Border Mail.

“I think they need some help,” she said of whoever killed the animals. “We have no idea why anyone would do that, especially to something as gentle as a platypus.”

Cook believes the culprit wanted someone to find the animals for some reason. The animals were likely trapped in a nearby river, killed and then dumped in the park, according to officials. Cook said it’s uncertain if the animals were beheaded while they were alive or after their deaths.

“Why take the heads?” she asked in an interview with The Guardian. “We still don’t know what they’ve done with the heads. These are just gentle little fellas who do no harm.”

The platypus is a semiaquatic egg-laying mammal native to eastern Australia. It lives in streams and feeds on what’s available there, including worms, insect larvae and freshwater shrimp.

It’s a crime to harm native animals in New South Wales. Penalties include a fine of up to $11,000 and up to six months in prison.