Story highlights Center for Biological Diversity pledges $15,000, raising reward total to $25,000

A man searching for shed deer antlers stumbled across the carcasses of four bald eagles; officials discovered nine more

U.S. Fish and Wildlife Service officials are still investigating the cause of death at a forensics center in Oregon

(CNN) After the discovery of 13 dead bald eagles in rural Maryland, animal protection groups have pledged up to $25,000 as a reward for information leading to an arrest and conviction, according to the Center for Biological Diversity.

The activist organization said Wednesday that it is offering up to $15,000. Three other groups, along with the U.S. Fish and Wildlife Service, have pledged a total of $10,000, the federal agency said.

Four of the dead eagles were found by a man who was searching a field for shed deer antlers Saturday near Federalsburg, in Maryland's Eastern Shore near the Delaware border, according to Maryland Natural Resources Police spokeswoman Candy Thomson.

He called the state agency, and wildlife officers found nine more carcasses after searching the area, Thomson told CNN.

It is "the largest die-off in one area in Maryland in 30 years," Thomson said, citing an incident in which eight bald eagles were found dead of suspected poisoning, though testing did not provide conclusive answers.

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