The reward money has gone up by $15,000 for any information about how 13 eagles ended up dead in a field in rural Maryland.

The Center for Biological Diversity said it is offering the money, which ups the reward total to $25,000 with contributions from the Humane Society of the United States and other groups.

The reward is part of a state and federal investigation after the bald eagles were found dead Saturday afternoon. A man, who said he was out looking for antlers that deer might have shed, came across what he initially thought was a dead turkey in a field on a farm in Caroline County. He discovered four dead bald eagles, and authorities were called.

When officers arrived, they found nine more dead bald eagles in the field on Laurel Grove Road in Federalsburg.

It was not immediately clear what had caused the birds to die, but there were “no obvious signs of trauma with these birds,” according to Candy Thomson, a Maryland Natural Resources police spokeswoman.

One theory is poison. Officials think someone may have sprayed a new chemical on a field that adversely affected the birds. Or someone may have used poison to kill rodents; if the rodents died outdoors and the eagles consumed their carcasses, the birds could get sick, too.

The discovery of the 13 dead bald eagles was the largest single incident in decades for the state, officials said. At least three of the birds were mature, with the signature white heads and brown bodies. Two of the birds were close to being mature birds, officials said, and the rest were considered immature birds with no white feathers.

Maryland’s natural resources police and the U.S. Fish and Wildlife Service are investigating the incident. The carcasses have been sent to the National Fish and Wildlife Forensics Laboratory in Ashland, Oregon, for more analysis and a necropsy. A finding on the cause of death could take weeks.

“These 13 bald eagles deserved better than to be killed,” said Catherine Kilduff, a lawyer at the Center for Biological Diversity in a statement.

“If they were poisoned or shot, the heartbreaking deaths of these 13 bald eagles is a crime,” she said. “Those responsible need to be caught and prosecuted.”

Thirty years ago, officials said, eight bald eagles were found dead in Maryland. Officials said they believe those birds may have been poisoned. And two years ago, two bald eagles were shot and killed in a week in Montgomery County.

Bald eagles are no longer on the endangered species list, but they are considered a protected species. They have been making a comeback, said wildlife experts. With the encroachment of development, they have gone from consuming as much fish as they want to being more scavengers and are often found eating roadkill from highways or at landfills, according to experts.

It is illegal to shoot eagles without a permit from the U.S. Department of the Interior.