Update June 21, 2018: A federal lab report determined that the eagles died after ingesting carbofuran, a chemical that has been banned in the U.S. since the mid-1990s. It's unclear who poisoned the eagles and why.

Update March 10, 2016: Investigators say the 13 eagles did not die of disease or other natural causes. Officials are treating the case as a possible criminal act.

It's a whodunit for the animal kingdom: State and federal wildlife officials are trying to find out what happened to 13 bald eagles that were discovered dead on Maryland's Eastern Shore on Saturday.

The birds of prey were found on a farm in rural Caroline County, about 90 miles (145 kilometers) east of Washington, D.C. and just west of the 3,800-acre Idylwild Natural Area. A man looking for shed deer antlers found some of the birds and phoned state officials, who then found several more.

"We don't know the cause of death yet and are asking the public for help with information," says Catherine Hibbard, a spokesperson for the U.S. Fish and Wildlife Service, which is investigating the case along with the Maryland Natural Resources Police.

Investigators are working the scene and are sending the bird carcasses to the federal forensic ornithology lab in Ashland, Oregon, to determine cause of death. Hibbard says it's too early to speculate on how the birds died but says its highly unusual for that many eagles to be found dead in such a small area.

"Our special agent has never seen this many dead eagles in eight years on the job," says Hibbard.

In recent years, officials have found a few eagles killed on the Eastern Shore from poisons that were put out by landowners to kill foxes or other animals, she adds. Eagles that scavenge on the poisoned carcasses can take up the poisons themselves, sometimes to lethal effect.

"Never have we seen this many eagles involved," Hibbard says, stressing that the investigation is ongoing.

The national symbol of the U.S., bald eagles were nearly wiped out by hunting, pesticides, and habitat loss in the 20th century. However, they have rebounded in recent decades thanks to strict protections and banning of DDT, which caused their eggshells to be too thin. Bald eagles were officially removed from endangered and threatened status in the U.S. in 2007, although they are still protected under the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act.

If people are ultimately convicted of causing the death of the eagles, they could face a fine as high as $100,000 and prison time up to one year as a result of those acts.

Anyone with information about the eagles is being asked to call the USFWS at 410-228-2476 or Maryland's investigators at 800-628-9944. A reward (raised to $25,000 as of March 10) is being offered for information that leads to a conviction.