MAMMOTH HOT SPRINGS, Wyo. -- The National Park Service has initiated a criminal investigation of the Tuesday release of 52 bison held for possible quarantine in a pen at the Stephens Creek facility in Yellowstone National Park, according to a news release.

"This is an egregious criminal act that sets back bison conservation," Yellowstone National Park Superintendent Dan Wenk said.

"It delays critical ongoing discussions about a quarantine program and the transfer of live Yellowstone bison to tribal lands," Wenk said. "The park is aggressively investigating this incident."

Park staff are working to recapture the bison, but have not been able to locate any as of Wednesday afternoon.

The park says the missing bull bison were being held in two separate pens: 24 were in confinement since March 2016; and 28 were held since March 2017.

Some person or persons entered the Stephens Creek facility, which is permanently closed to the public, and cut fencing.

The bison were being held and tested for brucellosis at Stephens Creek as part of a plan being considered to establish a quarantine program to augment or establish new conservation and cultural herds of disease-free plains bison. The program also would enhance cultural and nutritional opportunities for Native Americans, specifically for the Fort Peck Tribes in Poplar, Mont.; reduce the shipment of Yellowstone bison to meat processing facilities; and conserve a viable, wild population of Yellowstone bison.

Brucellosis is a bacteria-caused, contagious disease that primarily affects cattle, bison and other ruminant animals. It also affects humans. Brucellosis can cause cattle to abort. In humans, it is known as undulant fever because of the severe intermittent fever
accompanying infection, according to the U.S. Department of Health's Animal, Plant and Health Inspection Service.

U.S. Department of Interior Ryan Zinke said the Fort Peck Tribes have been working with park, Montana and the Animal and Plant Health Inspection Service for years to repatriate these bison.

"The criminals who broke into a national park facility to release these bison put at risk the safety of the animals that are now at risk of being culled and our park rangers who are rounding them up," Zinke said.

Anyone with information about this incident is encouraged to call the Yellowstone National Park Tip Line at (307) 344-2132 or email For more information, visit