The US government announced on Feb. 21 it was ending protection for this group of gray wolves, and that decision became effective on March 28.

Since then, conservation groups said in the suit, dozens of gray wolves have been killed in the three states.

"Wolves have not yet recovered," said Louisa Willcox of the Natural Resources Defense Council, which was among those groups seeking renewed federal protection for the species.

"Biologically, you need several thousand wolves in connected populations between Yellowstone (national park in Wyoming, Montana and Idaho) and Canada to achieve what scientists and geneticists believe is true recovery," Willcox said by telephone. "This plan calls recovery good at 300 animals."

The government defended its stance. "We believe it was time to de-list the Rocky Mountain population of the gray wolf, and we stand by that," said Sharon Rose of the Denver office of the US Fish and Wildlife Service. Rose declined further comment, saying she had not seen the lawsuit.

Once plentiful in the 48 contiguous US states, gray wolves were eradicated from the northern Rocky Mountain region and southwestern Canada by the 1930s. The species was listed as endangered in 1973; 66 wolves were re-introduced to the area in 1995.

The endangered species law is aimed at preventing extinction and protects species and their habitats.

By February of this year, there were 1,513 wolves in Montana, Wyoming and Idaho, including 107 breeding pairs, and wolf populations in these states grew by 24 percent annually since they were re-introduced, the US Fish and Wildlife Service said when the de-listing decision was announced.

Since the de-listing, the three states have taken over responsibility for managing wolf populations.

The lawsuit was filed by Earthjustice on behalf of NRDC, Defenders of Wildlife, Sierra Club, Center for Biological Diversity, The Humane Society of the United States, Jackson Hole Conservation Alliance, Friends of the Clearwater, Alliance for the Wild Rockies, Oregon Wild, Cascadia Wildlands Project, Western Watersheds Project and Wildlands Project.