BILLINGS — Montana Gov. Steve Bullock has blocked for now the impending slaughter of hundreds of Yellowstone National Park bison over disease concerns until a temporary home can be found for 40 of the animals wanted by an American Indian tribe.

Bullock prohibited the transfer of any park bison to slaughter in a letter to the park obtained Wednesday by The Associated Press. The Jan. 19 letter was not previously publicized and invoked an executive order used by Bullock’s predecessor, fellow Democrat Brian Schweitzer, to stop slaughter shipments in 2011.

About 200 bison have been captured in recent weeks attempting to migrate from the park into Montana.

Federal and state officials aim to kill up to 1,300 bison this winter using hunting and shipments to slaughter. It’s an attempt to drive down the size of the park’s herds, which currently number about 5,500 animals.

The killings are carried out under a state-federal agreement meant to prevent the spread of the disease brucellosis from bison to livestock.

Last year, the park kept alive 40 captured bison for future relocation to northeast Montana’s Fort Peck Indian Reservation — part of a faltering effort to establish new herds outside the park and reduce Yellowstone’s annual slaughters.

Those plans stalled after state veterinarian Marty Zaluski said Montana law prohibits the animals from being moved outside the Yellowstone area, despite past relocations of park bison to Colorado for a federal research program.

Months or years of additional tests are needed to certify the animals are disease free, state livestock officials said Wednesday.

The 40 bison already have tested negative for brucellosis multiple times, officials said, but there’s worry the disease still could appear, Zaluski said.

Yellowstone Superintendent Dan Wenk had notified the state that the 40 animals wanted by the Fort Peck Assiniboine and Sioux tribes likely would be slaughtered to free up space at the capture facility where they’ve been held since last March.

Bullock’s prohibition on moving park bison “saved those bison from being shipped to slaughter. There’s no doubt about that,” Wenk told The AP.

“In the long term, we would still like to find a way to get bison to Native American tribes,” he added.

Bullock spokeswoman Ronja Abel said the governor’s move to block slaughters would stay in effect until a resolution is found for the 40 bison. She said state and federal officials were investigating if the animals can be kept temporarily at a Department of Agriculture research facility just north of the park.

Once that issue is resolved, the park plans to begin shipping other bison to slaughter. Meat from slaughtered animals is distributed to American Indian tribes across the region.

“We recognize the park’s need to cull their herd,” Abel said. “We are trying to work toward a solution as fast as possible and hopefully we can come to a resolution by early next week.”

No transmissions of brucellosis from bison to livestock have been recorded. That’s in part because thousands of migrating animals were killed over the past several decades as soon as they left the park.

Jonathan Proctor with Defenders of Wildlife said Bullock’s slaughter prohibition was great news. But he said it did not negate the state’s resistance to moving bison out of the Yellowstone area for restoration elsewhere.

“We still need a real long-term solution and the Fort Peck Reservation provides that solution,” Proctor said.