The birds - three adults and one juvenile - were found last week floating in a lake on private property, said Sgt. Jennifer Maurstad with the Washington Department of Fish and Wildlife. A wildlife officer kayaking on the lake for work spotted the birds and reported them.
Investigators believe at least three of the birds were killed using a small-caliber rifle and were likely perched in trees before falling to their deaths, officials said.
"Normally when eagles are poached, it's for financial reasons, because their feathers and talons are worth money," Maurstad said, "but in this case, they were just left for dead."
Bald eagles are protected under state and federal law. The penalties for killing one include fines of up to $3,000 per eagle, in addition to 90 days in jail.
"Even though their numbers have increased, it's still illegal," Maurstad said. "It's an egregious act. It's the bird that is our national symbol, so just very brazen."
Hardware store owner Dan Meeks, who displays an American flag outside his shop and has two eagle tattoos on his left arm, was nearly brought to tears upon hearing the news of the eagles' killing.
"Hearing that these eagles were killed here in Granite makes me sick," Meeks said. "(I) swallow hard I'm mad."
"People talk in Granite Falls," he continued, "so if the kids that shot those are from around here, people in town will find out."
Investigators aren't sure if the birds were shot by adults or kids, but hope the larger reward - thanks to a $10,000 contribution from the Stillaguamish Tribe - helps.
"It's unfortunate," she said. "It's a mystery why you would just shoot them and leave them."