The West Nile Virus is killing the bald eagle, at least that's what Utah wildlife officials say most likely caused the deaths of dozens of bald eagles in December.

According to officials, 27 bald eagles have died since Dec. 1, and six are receiving treatment at a wildlife facility, the Associated Press reported. 

The Utah Division of Wildlife Resources said laboratory tests on the birds suggests the cause of death was the West Nile Virus. The eagles contracted the virus when they ate grebes, a type of diving bird, that were already infected, the DWR said in a statement obtained by the AP.

Up until now officials were baffled as to what was killing the bald eagles, America's national bird. Researchers were perplexed by how far the sick birds were found from each other, and by their strange symptoms, The Washington Post reported. Bald eagles in Utah usually die from a broken neck or wing from being hit by a car or accidentally flying into a power line or fence.

Leslie McFarlane, coordinator of the wildlife disease program for Utah's DWR, told The Washington Post that all of the bird's symptoms were the same. The eagles' wings were paralyzed, their legs were so weak they could not stand, along with body tremors and seizures.

"This is really concerning to us," MacFarlane told The Washington Post. The deaths were "very unusual."

The West Nile Virus was previously considered as the cause of the deaths because the symptoms were similar. But MacFarlane ruled it out because during this time of year there aren't enough mosquitos in Utah to infect the eagles, The Washington Post reported.

The DWR said the infected birds are not harmful to humans and that the eagles should be safe because water birds like grebes will begin to migrate, the AP reported.