BILLINGS, Mont. -- Gray wolves in the Northern Rockies will be removed from the endangered species list, following a 13-year restoration effort that helped the animal's population soar, federal officials said Thursday.

An estimated 1,500 wolves now roam Idaho, Montana and Wyoming. That represents a dramatic turnaround for a predator that was largely exterminated in the United States in the early 20th century.

"We're confident the wolf has a secure future in the Northern Rocky Mountains," said Interior Assistant Secretary Lyle Laverty in a statement. Federal officials planned a news conference later Thursday.

The loss of federal protection allows the three states to move forward with plans allowing hunters to target the animals, possibly as soon as this fall.

Environmental groups said Thursday they will sue the federal government to keep the animal listed.

"The enduring hostility to wolves still exists," said Earthjustice attorney Doug Honnold, who is preparing the lawsuit. "We're going to have hundreds of wolves killed under state management. It's a sad day for our wolves."

State officials have been eager to gain greater control over managing wolves, which have increasingly preyed on livestock as they expand into new territories. Since the late 1980s, approximately 700 wolves have been killed by ranchers and state and federal wildlife agents in response to livestock conflicts.