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Police in the US state of Oregon are investigating the poisoning of eight grey wolves found dead by officials earlier this year.
Five wolves were discovered near Mount Harris in February, followed by another three later.
Tests later confirmed that a "poisonous substance" had killed the wolves.
Authorities are asking for help from public and conservation groups are offering a $26,000 (£19,600) award for information leading to conviction.
In a statement, Oregon State Police said that the initial batch of five wolves - all of whom were collared members of the same pack - were found dead along with a nearby magpie on 9 February.
Just over a month later, on 11 March, officials responding to a 'mortality signal' from a wolf collar discovered another dead wolf, a skunk, and a magpie. Two more wolves were found dead in April and July.
Toxicology reports confirmed that all the wolves had been poisoned.
The police statement said that investigators have "exhausted leads" in the case and are seeking information from the public.
Brooks Fahy, the executive director of Oregon-based wildlife advocacy organisation Predator Defense, said that the poisonings were a "cowardly and despicable act".
Grey wolves were once nearly wiped out across the contiguous US.
The Endangered Species Act of 1974 created federal protections that saved the species from extinction and led to sustained population recovery efforts.
But the animals were delisted by the Trump administration last year and management of the species has since fallen to the states. The Biden administration is reportedly considering relisting grey wolves on the Endangered Species List.