A mystery disease appears to be killing off bald eagles in Utah, but the state’s wildlife experts aren’t quite sure why.

Sixteen birds have been either found dead or have died in rehabilitation from Dec. 1 through Friday morning, says Leslie McFarlane, the wildlife disease program coordinator for the state’s Division of Wildlife Resources. The symptoms are similar, she says: paralysis of the wings, weakness in the legs and feet making it difficult for the birds to stand, body tremors, and, ultimately, seizures.

“This is really concerning to us,” says McFarlane. She has been program coordinator for 10 years and describes the recent deaths as “very unusual.”

The annual number of bald eagle deaths in Utah can range from 4 to 40, McFarlane says. But those deaths are almost always associated with injuries, such as a broken neck or wing from flying into a vehicle, fence or power line. The symptoms noted in the recent spate of deaths—and the broad geographical area in which they have cropped up—are what has officials concerned.

State officials have ruled out lead poisoning. And while the symptoms are similar to those caused by the West Nile Virus, McFarlane said, there are few mosquitoes to transmit the virus in Utah at this time of year.

Eagles in the state are typically not year-round residents. An estimated 750 to 1,200 can be found in Utah during the winter months, having come down from Alaska and Canada.

“We don’t have a whole lot of nesting bald eagles here,” McFarlane says.

Almost all of the carcasses have been sent for further study to the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wisconsin. McFarlane said that she is hoping the center will respond this week with some clues as to what’s afflicting the population.