The Humane Society of the United States is offering a $5,000 reward for information on who illegally killed three wolves in northern Minnesota, upping the ante in the ongoing investigation announced this week by the U.S. Fish and Wildlife Service.

The reward, for information leading to the identification, arrest and conviction of the people responsible, would be in addition the $2,500 offered by the wildlife service.

In what conservation officials are confident is a case of poaching, carcasses of three wolves were found dumped in a ditch along Hwy. 8 about 35 miles southeast of Grand Rapids, federal authorities said Thursday. The discovery was reported on Jan. 22 to a state Department of Natural Resources poachers tip line.

The gray wolf is listed as “threatened” under the federal Endangered Species Act, meaning they cannot be hunted except in defense of human life. A conviction for each violation could result in up to six months in prison and a fine of up to $25,000.

“There is no excuse for deliberately killing three members of a threatened species and discarding the animals like litter. ...” said Christine Coughlin, Minnesota director for the humane society. “We’re hopeful this reward will bring forward anyone with information about this heinous crime.”

Conservation officials said that it appeared the animals were killed elsewhere then moved to the ditch. They carried marks on their necks that indicated they had been caught by snares, but the cause of death is under investigation.

A judge in 2014 reinstated Endangered Species Act protections for gray wolves in Minnesota, Wisconsin and Michigan and placed the animals under protection of the Fish and Wildlife Service. They were removed in January 2012 from the endangered list, which briefly allowed for hunting seasons.