US 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 (16 km) 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 US 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."