Thirteen bald eagles that were found dead near a farm in Maryland in late February did not die of natural causes, according to the U.S. Fish and Wildlife Service. After receiving necropsy results, wildlife officials said 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. "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 and may have been poisoned, 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 tells 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 says.

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 hasn't been ruled out, 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 is again poisoning. The birds could have died from a chemical sprayed on a field or could have eaten rodents that had eaten pesticides. If poison is the cause, it could be benign or it could be purposeful and malicious, Thomson said. Investigators 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 are also looking into a possible natural cause for the extraordinary number of deaths. They're 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 says. "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.

There's a $10,000 reward being offered for information leading to the resolution of the Maryland incident. Anyone with information is asked to call 410-260-8888.

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.