Five wolves were initially found dead, and another three were found after. Testing found that a "poisonous substance" had killed the wolves.
A $26,000 reward has been offered from the public and conservation groups for information leading to a conviction of the individual who poisoned the animals.
The wolves were all members of the same pack. A magpie was also found dead near where the wolves were found on 9 February.
During the second discovery, a skunk and another magpie were also found dead.
According to a police statement, the investigators had "exhausted leads" in the case, which is why they're seeking help from the public.
Oregon Public Broadcasting reported that the poisonings occurred as tensions between ranchers, their supporters, and wildlife rights advocates increased due to livestock deaths. According to state records, 29 cows, 19 sheep, nine goats and one dog have been killed in the state by wolves.
Amaroq Weiss, the senior wolf advocate with the Centre for Biological Diversity, told OPB that the livestock deaths attributed to wolves were only a small fraction of the number of livestock that die each year.
“One one-thousandth of Oregon’s livestock are lost each year to wolves,” she said. “It’s a very small percentage.”
She said that killing wolves, particularly a pack's alpha males, may actually make the situation worse, as it leaves female wolves with the burden of both hunting for the pack and rearing cubs. As a result, rather than hunting deer or elk, the female wolves may opt for easier to catch prey, like livestock grazing in a field.
The BBC spoke with an Oregon State Police official who said that there are several charges the individual could face.
If the person took or possessed a wolf, they could face felony charges and a prison sentence of up to five years and $125,000 in fine. They could be charged for placing a toxic substance in a place where wildlife can access it, which is punishable by up to one year in jail and fines of more than $6,000. Civil damages could also apply, to the amount of $7,500 for each wolf killed.
Charges will be determined based on the circumstances of the case.
Grey wolves were protected by the Endangered Species Act of 1974 after they were nearly wiped out. The population has since bounced back to a sustainable level. However, under former President Donald Trump's administration, the wolves were delisted from protection.
The Biden administration is reportedly considering adding the wolves back onto the list to protect their population.