BILLINGS — About 10 bald eagles from around Montana have shown up with heightened levels of mercury in recent months — two have died — and no one knows exactly why.

The unusual development is prompting a study this summer looking at the possible sources of mercury and whether more birds may be at risk.

The eagles brought into the Montana Raptor Conservation Center in Bozeman since December have had varying levels of mercury. They have come from points throughout Montana including Lincoln, Red Lodge, Fort Benton and near Dillon.

“We’re getting them from pretty much all over,’’ said Radell Key of the Montana Raptor Conservation Center.

Several months ago, an eagle was brought in that appeared to show clinical signs of lead poisoning. The bird tested negative for lead but positive for mercury. Since then, officials have been testing bald eagles for mercury, something they hadn’t done in the past.

It’s too early to tell whether the 10 to 12 bald eagles with heightened levels of mercury is simply a result of investigators looking for it or whether something else is going on.

“We don’t know,’’ Key said. “It’s just something we’ve started looking at.’’

Although Montana’s overall bald eagle population continues to do well, those found with mercury “raise some red flags,’’ said Kristi DuBois, chairwoman of the Montana Bald Eagle Working Group and native species coordinator for Montana Fish, Wildlife and Parks.

“The fact that there are a few birds that died may be a warning sign for us,’’ DuBois said. “The key is to find where it’s coming from.’’

Mercury is a widespread element that occurs naturally in certain types of soils and rocks. It’s also a pollutant associated with coal-fired power plants, mines, burning garbage and other human-related operations.

Once it’s in the water, mercury is typically converted to methylmercury by bacteria, which fish and other animals absorb and retain in their muscle tissue. When an eagle, osprey or other animal along the food chain eats that fish, it gets the poisonous mercury along with its meal.

One of the keys with the latest cases in bald eagles will be figuring out whether the bald eagles were residents, migrants or visitors from Canada, DuBois said.

“It’s hard to say whether they were Montana eagles or not,’’ said DuBois, whose agency has agreed to pay for a portion of the study.

The study will examine blood, feathers and carcasses from bald eagles and osprey in southwestern Montana. Because newly deposited mercury is more readily converted to methylmercury, contamination in the upcoming investigation could indicate recent exposure and may point to pollution ’’hot spots,’’ according to an outline of the study.

The discovery comes as the U.S. Fish and Wildlife Service is proposing to remove bald eagles from the endangered species list.

The bird’s recovery, heralded as a national conservation success story, is largely attributed to a 1972 ban on the pesticide DDT, which caused eagles’ eggs to thin and break, and increased efforts to protect its habitat, crack down on poaching and implement captive breeding and reintroduction programs.

In the lower 48 in 1963, there were an estimated 487 active nest sites. Today, there are more than 7,000 breeding pairs. In Montana, where in 1978 there were just 12 breeding pairs, today there are nearly 400.