An insecticide is believed to have killed almost 200 native birds in northeast Victoria.
Dead wedge-tailed eagles found near Violet Town in August, led the state's environment department to find more, along with hawks and falcons, on a nearby property.
They have since found up to 200 dead native birds in the area, including 25 wedge-tailed eagles.
Tests of six eagles have detected an insecticide used to control mites.
The same agricultural chemical has been found in the carcasses of animals suspected of being used as bait, with the department believing it may have caused all of the bird deaths.
But they aren't sure whether the poisoning was an accident.
"It remains unclear if these birds were deliberately poisoned, however given the large number of birds found nearby, it's a possibility," environment department compliance manager Andrew Dean said.
Raids have also taken place in recent weeks at properties in Shepparton East and Goomalibee.
"All evidence collected will be forensically analysed, including the carcasses and chemicals seized, which may take some time."
Native birds are protected under the Wildlife Act and deliberately killing them can result in a fine of up to $39,652 or up to two years in prison.
Australian Associated Press