COMMERCE CITY, Colo. - On the prairie where her ancestors once blanketed the landscape, a bison yearling lifted up her muzzle and pirouetted before bounding off in the shadow of the Rocky Mountains.

Hunted to the brink of extinction by settlers in the 19th century for sport or to make room for railroads, farms and ranches, North American bison, more commonly called buffalo, are being returned to the high plains of Colorado.

U.S. wildlife managers on Saturday released 16 bison -- three bulls and 13 cows -- into the Rocky Mountain Arsenal Wildlife Refuge as part of an effort to return genetically pure, wild bison to the short-grass prairie where they once numbered in the millions.

The site, 10 miles from downtown Denver, was once a chemical weapons plant.

"Bison are the keystone species of the prairie ecosystem," said Dean Rundle, a refuge supervisor for the U.S. Fish and Wildlife Service.

Four of the reintroduced cows are pregnant, so wildlife managers will be able to control and monitor a breeding population of the purebred shaggy creatures, Rundle said. The agency is spreading wild bison to six refuges across the western United States.

Before a crowd of onlookers and prodded by two wranglers, the bison bounded from the tractor-trailer in which they made the 15-hour trek from the National Bison Refuge in Montana, home to one of the last pure buffalo herds.

As the American West was settled, wild bison were bred with domestic cattle. The result was that few bison herds remained free of cattle genes.

Lee Plenty Wolf, a Lakota Indian spiritual leader from the Pine Ridge Indian Reservation in South Dakota, said he made the trip to honor their legacy to his people, who consider the buffalo sacred.

"In mythological times, the buffalo brought our people out of the Black Hills," he said before blessing the site with the feathers of a spotted eagle and a drum that symbolizes the heartbeat of the Earth. "They helped us survive on this continent by giving us food, shelter and clothing."

As Plenty Wolf finished his prayer chant, the three newly released bulls, each weighing 1 ton (900 kg), turned their heads away from the crowd and disappeared over the ridgeline.