Four bald eagles have been shot dead in Minnesota since April, prompting wildlife officials to open forensic investigations and offer a reward for information leading to convictions for killing the federally protected birds.

The U.S. Fish and Wildlife Service (FWS) said Tuesday that an eagle was apparently shot in October on the Leech Lake Indian Reservation — the fourth such shooting known to have occurred this year. In April, three others were found shot along a county highway near Cook, Minn., and a separate criminal investigation is underway. All four carcasses were sent to the FWS laboratory in Ashland, Ore., for forensic analysis on the type of bullet and other information, wildlife officials said.

Now a reward of up to $2,500 for information that leads to the conviction of any responsible party is being offered, officials said Tuesday.

Officials with the wildlife service refused to discuss possible motives, suspects or other information developed in the investigations so far.

Such incidents are unusual in Minnesota. There have been 67 eagle shootings since 2001 in the eight states that make up the wildlife service’s Midwest region, said Deanne Endrizzi, a biologist with the agency.

Such killings almost always result in investigations because bald and golden eagles are among the birds protected by federal laws that go back to 1900.

Once listed as endangered, when their numbers dwindled to an estimated 417 nesting pairs in the lower United States, eagles have recovered to about 70,000 individuals in North America, according to some estimates.

Minnesota has one the largest eagle populations in the Lower 48 states, with about 10,000 pairs, according to the Minnesota Department of Natural Resources (DNR).

The University of Minnesota’s Raptor Center, which takes in dead and injured birds of prey, sees about 10 eagles a year with some kind of projectile injury, said Juli Ponder, executive director.

For example, in 2017, 10 out of 183 dead or injured eagles brought to the center had some kind of bullet injury.

And while that is a reflection of what’s happening to the birds in the environment, it is impossible to know the real number that are shot, she said.

Still, it is far less than the number of birds that die from accidental or unintentional injuries caused by humans, she said. While guns are the leading cause injury or death to the national bird, it’s not the ones pointed at eagles. About a fourth to a third of the adult eagles that the center studies had been poisoned by lead shot after eating animal carcasses or gut piles left by hunters, she said.

Intentional killings spark investigations and criminal charges by federal and state wildlife officials.

But other illegal animal killings have also resulted in criminal charges. One of the most notable cases in the Upper Midwest involved a father and son in Wisconsin’s Oneida County who in 2007 used poison to kill bobcats, coyotes, wolves, fishers and other species that preyed on the deer and game birds they routinely hunted on their land. In addition to many other species, the poison also killed eagles.

Penalties for killing eagles or other protected birds under the Migratory Bird Treaty Act include a maximum of two years imprisonment and $250,000 fine for a felony conviction, and six months imprisonment or $5,000 fine for a misdemeanor conviction.

Under the 1900 federal law, penalties include a maximum of five years in prison, and $250,000 fine for felony convictions, and a maximum $10,000 fine for civil violations.

Federal officials asked anyone with information about the recent eagle deaths to contact special agent Ron Kramer at the FWS Duluth office or the DNR’s Turn in a Poacher TIP line (800-652-9093).