By KEITH RIDLER

Associated Press

BOISE, Idaho (AP) -- Idaho officials have killed two bull bison that roamed out of Yellowstone National Park, citing concerns over the spread of brucellosis to cattle.

A spokesman for the Fremont County Sheriff's Office said one bison was killed Saturday and another Thursday in Island Park in southeastern Idaho. The area is about 15 to 20 miles west of the park.

In a statement the Idaho Department of Agriculture said the state's policy requires wild bison to be either killed or moved because of the possible spread of disease. Brucellosis can cause pregnant animals to miscarry their young. There have been no recorded instances in the wild of bison transmitting brucellosis to cattle.

Authorities said the bison killed Thursday was along the shoulder of U.S. Highway 20 near The Nature Conservancy's Flat Ranch. The bison killed Saturday was north of Mack's Inn.

The Nature Conservancy in a statement said the bison killed Thursday was shot by an employee of the Idaho Department of Agriculture and was not on land owned by the conservancy.

"Our staff and volunteers in Idaho are saddened about this unfortunate situation and the death of the bison," the group said in a statement that also noted its bison conservation efforts.

The Idaho Department of Agriculture did not return a call from The Associated Press on Sunday.

It's unclear when authorities last killed a bison in Idaho.

Darrell Geist with the Buffalo Field Campaign, an advocacy group based in West Yellowstone, Mont., said the last one he investigated involved a bull killed by the U.S. Department of Agriculture in August of 2009, also near the Flat Ranch.

He said bison could be entering Idaho a number of ways from Yellowstone National Park, including along Targhee Pass. He noted the region used to be a major bison migration corridor, and some bison are still drawn to it.

"It's their instinct to migrate, and there are some bison that still retain these ancient migratory routes, almost like a map," he said. "They know where to go and they find it again."

Copyright 2012 The Associated Press.