In the 1990s, 14 wolves from Canada quickly reproduced after they were released in Yellowstone National Park in north-west Wyoming.

BBC: Wyoming drops federal protection of gray wolves

But some ranchers claim that there were indeed wolf sightings just before the reintroduction program was started, due to a small migration of wolves from Canada into the reintroduction area.

CNN: Appeal of wolf removal rescheduled

They transplanted dozens of wolves to Yellowstone from Canada.

CNN: Gray wolf: Still endangered?

Environmentalists say the 30 wolves re-introduced into the United States from Canada in 1995 have successfully established packs and have grown in numbers more than tenfold.

CNN: Appeals court ruling protects wolves in Yellowstone