Last Saturday was a sad day for America’s national bird. Thirteen bald eagles were found dead on Maryland’s Eastern shore, prompting state and federal wildlife officials to speculate over what killed so many members of the once-endangered (but still federally protected) species. Now, CBS Baltimore reports that the current theory is that the birds of prey might have died after ingesting poisoned rodents or toxic farming chemicals.

Officials received a phone call from man named Glenn Breeding, who had discovered four deceased bald eagles while searching for deer antlers on his farm in rural Caroline County, according to KCBY CBS 11. Authorities found nine other casualties in the area.

Officials weren’t initially sure how the birds died and admitted that it's rare to find a die-off of this size at one location, National Geographic reports. And the bodies on the birds didn't seem to give them any obvious clues. According to The New York Times, the corpses didn’t show any signs of trauma, and that they were discovered near normally hospitable grounds for bald eagles: creeks, farms, and protected lands.

It won't be the first time bald eagles on the Eastern Shore have died from ingesting dead animals that had eaten poisons set out by landowners. But experts aren't ready to call it yet: Officials say they won’t know for sure how these birds were killed until necropsies are performed. In the meantime, awards are being offered to tipsters who can help solve the mystery of what The Washington Post is calling "the largest single die-off of bald eagles” Maryland has experienced in 30 years.

[h/t CBS Baltimore]