Federal authorities have determined how 13 bald eagles found dead on the Eastern Shore have died, but will only say publicly that they have ruled out natural causes.

The focus of the investigation is now on prosecuting anyone responsible for the deaths — Maryland’s largest single die off of the eagles in at least three decades.

Ruling out disease, which the eagles could have caught from chickens at the numerous poultry farms that dot the area or from migrating birds, was an important step in the case, U.S. Fish and Wildlife Service spokeswoman Catherine Hibbard said.