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. 
 
 Hibbard said in an email that while investigators have determined how the eagles died they won't release more information — including about any suspects — for fear of compromising the investigation. 
 
 The birds were found in February by a man roaming farm fields near Federalsburg looking for deer antlers. Maryland Natural Resources Police are working with federal authorities on the case and a $25,000 reward has been offered for information that leads to a conviction. 
 
 The birds showed no signs of trauma, indicating that they might have fallen prey to illness or poison. 
 
 iduncan@baltsun.com 
 
 twitter.com/iduncan ||||| The 13 bald eagles that were found dead last month on Maryland's Eastern Shore didn't die of natural causes, wildlife officials said Thursday, and authorities are offering a $25,000 reward to help find whoever killed them. 
 
 The birds showed no signs of trauma when they were found Feb. 20 on a farm and in the woods in the town of Federalsburg in what Maryland Natural Resources police said was the biggest die-off of bald eagles in the state in 30 years. 
 
 Investigators initially speculated that they might have eaten poisoned animal carcasses put out to control rodents, but the owner of the farm told NBC Washington at the time that he didn't use poison. 
 
 The U.S. Fish and Wildlife Service said Thursday that necropsies had also ruled out diseases, such as avian influenza, a major threat in the region because of its many poultry farms and migratory birds. 
 
 "Our investigation is now focused on human causes," said Catherine J. Hibbard, a spokeswoman for the agency, who said she couldn't release any more information to protect the investigation. 
 
 Bald eagles were removed from the endangered species list in 2007, but they're still listed as "protected," and killing one is a crime punishable by up to a year in prison and a $100,000 fine. 
 
 The Fish and Wildlife Service and Maryland Natural Resources police said a $25,000 reward has been posted for information leading to the conviction of the suspected wildlife offenders.
– A bald eagle mass murder? The US Fish and Wildlife Service says it has tested 13 bald eagles found dead near a Maryland farm last month and concluded that they were killed by humans, NBC News reports. An agency spokeswoman tells the Baltimore Sun that avian influenza and other diseases were ruled out in the state's biggest die-off of the birds in at least 30 years, and the investigation is now focused on "human causes" and bringing the offenders to justice. The birds showed no sign of trauma and the Fish and Wildlife spokeswoman says investigators know how they died, but that information will not be released to avoid compromising the investigation. Bald eagles are a protected species and the reward leading to the culprits in this case has been raised to $25,000.