Story highlights A necropsy shows the eagles didn't die of disease, other natural causes

Investigation now "focused on human causes," Fish and Wildlife Service said

(CNN) Thirteen bald eagles found dead last month in Maryland didn't die of natural causes. Authorities hint that humans may be to blame.

The U.S. Fish and Wildlife Service released the results of the necropsy done on the eagles, found February 20 near Federalsburg, in Maryland's Eastern Shore. The results indicate the eagles, which didn't show any outward signs of trauma, didn't succumb to disease or some other natural cause.

Ruling out a disease -- such as avian flu -- as the cause is important, the Fish and Wildlife Service said, considering the area's many poultry farms and migratory birds.

"Our investigation is now focused on human causes and bringing to justice the person(s) responsible for the death of these eagles," the Fish and Wildlife Service said in a statement.

But the service, which is working with the Maryland Natural Resources Police, didn't give out any more details in the case, citing the ongoing investigation. A $25,000 reward leading to the arrest and conviction of suspects has been offered.

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