A team of researchers is investigating a mysterious and deadly illness taking the lives of a treasured national icon: the bald eagle. To learn more, please visit the Wildlife Rehabilitation Center of Northern Utah at . NBC's Miguel Almaguer reports.

The tally of unexplained bald eagle deaths in Utah this month rose to 20 on Thursday as state wildlife officials looked for possible links to diseases suspected in a coinciding die-off of thousands of shore birds around the Great Salt Lake.

Since Dec. 1, state wildlife specialists have documented a growing number of bald eagles of varying ages succumbing to an unexplained ailment that crippled them with leg paralysis and tremors before they died.

The eagle deaths have been concentrated in the northern and central parts of Utah at a time when the federally protected raptors have migrated to their wintering grounds in the Rockies.

Necropsies, the animal equivalent of autopsy examinations, have yet to pinpoint what is killing the eagles, but scientists now believe a disease rather than a toxin is the culprit, said Leslie McFarlane, Utah wildlife disease coordinator.

"It appears to be more disease-related since we're seeing birds with neurological symptoms and enlarged hearts. That doesn't rule out all toxins, but it shortens the list of suspects," she said.

McFarlane said a recent die-off in Utah of eared grebes that began in November and has now killed thousands of birds may be tied to the deaths of eagles, which are known to prey on the small shore birds.

Avian cholera and another highly infectious bacterial disease - erysipelas - are suspected in the grebe die-off, which has led biologists to theorize that afflicted eagles may have contracted one of the two diseases by feeding on infected grebes, she said.

Avian cholera and erysipelas do not pose high risks to humana, according to the U.S. Geological Survey. The bacteria behind both diseases can survive long periods in the environment, can be transmitted from bird to bird or by consumption of contaminated water or food, a USGS report shows.

The largest previous recorded die-off from erysipelas in the United States of eared grebes and other birds occurred in 1975 in the Great Salt Lake, according to the USGS.

Preliminary testing of eagle carcasses by a national wildlife lab in Wisconsin has so far ruled out poisoning from lead ammunition used by hunters to shoot ducks and other waterfowl included in the diet of eagles, McFarlane said.

Bald eagles, the U.S. national symbol, were removed from the federal threatened and endangered species list in 2007 after they soared back from near extinction.