FEDERALSBURG, Md. -- Federal wildlife officials say 13 bald eagles found dead on Maryland's Eastern Shore did not die of natural causes.

U.S. Fish and Wildlife Service spokeswoman Catherine J. Hibbard said in a statement Thursday that necropsy results of the eagles discovered in Federalsburg on Feb. 20 ruled out disease.

CBS Baltimore station WJZ-TV reports that officials believe a human caused the deaths.

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Hibbard says eliminating the possibility of diseases such as bird flu is important because the area has numerous poultry farms and migratory birds.

She declined to release further details about how investigators believe the birds might have died.

A $25,000 reward is being offered for information that leads to an arrest and conviction.