Thirteen bald eagles died of something other than natural causes, Maryland Department of Natural Resources Police said.
Police said in a tweet they're investigating "human causes" after a lab report ruled out disease.
The dead birds were found last month around Federalsburg on the Eastern Shore. Authorities said at the time that the birds did not have any obvious signs of trauma.
Lab results: 13 bald eagles did not die of natural causes, including disease. Investigation turns to human causes & finding suspects.
— MD NRP (@MDNRPolice) March 10, 2016