SALT LAKE CITY — Bald eagles are dying in Utah – 20 in the past few weeks alone – and nobody can figure out why.

Hundreds of the majestic birds – many with wing spans of 7 feet or more – migrate here each winter, gathering along the Great Salt Lake and feasting on carp and other fish that swim in the nearby freshwater bays.

Earlier this month, however, hunters and farmers across five counties in northern and central Utah began finding the normally skittish raptors lying listless on the ground. Many suffered from seizures, head tremors and paralysis in the legs, feet and wings.

Many of the eagles were brought to the mammoth Wildlife Rehabilitation Center of Northern Utah, where Buz Marthaler – a longtime animal caretaker – and other handlers tried to save the birds. Within 48 hours most were dead.

“It’s just hard to have your national bird in your arms, going through seizures in a way it can’t control – when you can see it’s pain but don’t know what’s happening to it,” said Marthaler, 56, co-founder of the facility in Ogden.

“As a human being, you just have problems with that. And when you lose one, it just grabs your heart.”

State wildlife specialists are also baffled. For weeks, officials have sent birds for necropsies at the National Wildlife Health Center in Madison, Wis., hoping the results would offer clues.

They began to rule out obvious possibilities: The birds were not shot by hunters, and officials don’t believe the birds were poisoned. “There doesn’t seem to be anything suspicious in that regard,” said Mitch Lane, a conservation officer with the Utah Division of Wildlife Resources, who has responded to numerous reports of downed or sick eagles.

At first, the agency’s scientists guessed that the illness could be encephalitis, which is caused by the West Nile virus, but later ruled out that possibility. And although many sick eagles tested positive for lead, researchers did not think that it was killing the birds.

Officials suggest the eagle die-off is possibly connected to the deaths of thousands of eared grebes that began in Utah in November. Eagles are known to prey on the small shorebirds. Because the grebes are thought to have died from avian cholera, many scientists theorize that the eagles became sick from feeding on infected grebes. Officials still don’t know why the shorebirds became sick.

“We’re getting closer to an answer,” Lane said, adding that officials would meet this week to continue investigating the mystery.

Meanwhile, a new ailing bald eagle surfaces almost daily.

Scott Isaacson, 59, an attorney who lives in the town of Farmington, said he was feeding his chickens one night this month when he spotted an eagle on the ground under a cottonwood tree where he was used to seeing seven or eight birds perched in the branches. “I’ve never seen one on the ground,” he said.

He called wildlife officials, who told him to approach bird; if it was healthy, it would fly off. The bird skittered into a nearby pond.

“It was really sad to see this graceful creature, with its beautiful white head, its wings spread out in the water,” he said.

Wildlife officials later nabbed the bird, which was hissing and clawing as it was scooped up in a net.

Bald eagles were removed from the endangered species list in 2007. Utah officials say 700 to 1,200 winter here each year. “Everybody here loves the eagles,” Isaacson said. “Our school is named Eagle Bay Elementary and our church is the Eagle Shore Church.”