Curtis Yarlott, executive director of the St. Labre Indian School, said the animals were probably spooked by a thunderstorm over Memorial Day weekend and ran through a fence.
The bison, which provide meat for the school, were last seen on a neighboring property June 3, but constant rain made it difficult to round them up.
"We don't know where they're at and what they're up to," he said. "Probably no good."
Only 13 bison from the herd have wandered back to the ranch. Yarlott, who hired a plane to search for the missing beasts, said the animals could have been rounded up and taken to market.
Ranch manager Ray Orthman agreed. "I think somebody took them," he said. "They were tame. They'd follow you like dogs."
The St. Labre Indian School was established in 1884 to educate and help the Northern Cheyenne. The ranch was donated to St. Labre almost nine years ago, and the buffalo were donated soon after.
