By Laura Zuckerman

(Reuters) - A number of unexplained bald eagle deaths in Utah when hundreds of the government-protected birds have migrated to wintering grounds in the central Rocky Mountains has wildlife officials worried.

The officials said on Tuesday at least four bald eagles have died and another is close to death. Wildlife specialists said the deaths began December 1 and point to either an outbreak of disease or exposure to some unknown toxin among the raptors in Utah.

The ailing eagles, which have been reported everywhere from front yards to river banks, have died within days of being grounded by such symptoms as leg paralysis and tremors.

"We have no idea what this is or isn't, so we can't rule anything out at this point. But we're taking it very seriously," Leslie McFarlane, Utah wildlife disease coordinator.

McFarlane said the state has never before tracked the deaths of so many bald eagles in rapid succession over a wide geographic area, and that she is not aware of a similar event elsewhere.

The bald eagle, the U.S. national symbol, was removed from the federal threatened and endangered species list in 2007 after their numbers came back from near-extinction. A key culprit in the plummeting eagle populations in the 20th century was the pesticide DDT, which was banned in the United States in 1972.

Eagles still are safeguarded by two federal laws, which make it illegal to kill or injure them without a special permit.

Bird carcasses found in Utah have been sent to a U.S. lab for testing, with results expected in coming weeks.

Four of the affected eagles, including one found over the weekend that was still clinging to life, were tended to at the Wildlife Rehabilitation Center of northern Utah in Ogden.

"We haven't seen anything like this before, with so many coming in from different areas with the same symptoms and in a short time span," said DaLyn Marthaler, the center's executive director.

Marthaler said the facility, which treats up to 400 eagles and other raptors each year, fears death is imminent for the adult male eagle still in its care.

"Words can't describe what it feels like to see them struggling and to know there's nothing that can be done until we know more about what's causing this," she said.

(Reporting by Laura Zuckerman, editing by Steve Gorman and Kenneth Barry)