Thirteen bald eagles that were found dead near a farm in Maryland in late February 2016 did not die of natural causes, according to the U.S. Fish and Wildlife Service. Officials now know the birds were poisoned.

According to The Washington Post, a U.S. Fish and Wildlife Service report says the birds ingested a pesticide called carbofuran. The pesticide is essentially banned from the U.S. partly due to the fact that it is lethal to birds, The Post reports.

Investigators believe the eagles fed on a raccoon that may have been purposely poisoned. Even if the eagles weren't the target, Fish and Wildlife Service special agent John LaCorte told the Post that someone illegally used carbofuran and added to a wildlife poisoning "epidemic on the Eastern Shore."

Searching for answers

Not long after wildlife official first received necropsy results in 2016, they were able to rule out natural causes, including disease.

"Our investigation is now focused on human causes and bringing to justice the person(s) responsible for the death of these eagles," said USFW spokeswoman Catherine Hibbard in a press release at the time. "We cannot release further details about the cause of death as such information may compromise the ongoing investigation."

The birds showed no sign of trauma, according to preliminary information released by officials when the birds were discovered. The incident is the state's largest bald eagle die-off in three decades.

A man was in a field in Federalsburg, near the Delaware border, on Feb. 20 looking for antlers that deer had shed when he discovered what he thought was a dead turkey. On closer inspection, he realized it was an eagle. He walked through the field and found three more eagles. The man called Maryland Natural Resources Police to report his findings.

Officers arrived and walked the field, finding nine more dead eagles, Maryland Natural Resources Police spokesperson Candy Thomson told MNN. "That would make it the largest single bald eagle die-off in Maryland in 30 years."

Three of the eagles were mature (meaning they had their white feathers), two were close to mature, and the rest were immature, Thomson said.

dead bald eagle found in MarylandThis is one of the 13 bald eagles found by Maryland Natural Resources Police in Federalsburg, Maryland. (Photo: Maryland Natural Resources Police)

The birds had not been dead for long, as they had not been attacked by scavengers. There were no apparent signs of trauma, although that wasn't ruled out at the time, according to Thomson.

"Certainly we have had eagle deaths where we haven’t discovered pellets in the bird until we had X-rayed it," she said.

The U.S. Fish & Wildlife Service was called in for a joint investigation and the eagles were sent to the USFW's Wildlife Forensics Laboratory in Ashland, Oregon, to determine the cause of death, according to Hibbard.

"Both our folks and the federal agents have never seen anything like this," Thomson says.

About 30 years ago, eight dead eagles were found in Maryland. They were believed to have been poisoned.

The working theory this time was again poisoning. Investigators thought the birds could have died from a chemical sprayed on a field or could have eaten rodents that had eaten pesticides. They went back out and walked the field inch by inch to make sure there were no other dead eagles or any other dead species and they found nothing.

Investigators also looked into a possible natural cause for the extraordinary number of deaths. They asking residents if they saw or heard anything unusual.

"This is a big deal. It's not only the number of eagles. It's the fact that eagles are beloved," Thomson said. "People have a pride in the fact that we brought eagles back. People love to watch eagles. They gather at the mouth of the Susquehanna River to watch the eagles fish. This area where the eagles died is wonderful eagle habitat. There's a wildlife management area right there. This is not what you expect in a place like this."

Bald eagles are no longer an endangered species but they are still federally protected, according to the U.S. Fish & Wildlife Service. Maximum fines for harming a bald eagle are $100,000 and up to one year in prison.

This story has been updated since it was originally published in February 2016.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

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