A coalition of environmental and American Indian groups sued two federal agencies Monday to stop the mass slaughter of bison that migrate outside Yellowstone National Park in search of food.

During the last decade, federal agencies working with the state of Montana captured and shipped to slaughter more than 3,300 bison to prevent the spread of an animal disease to cattle.

Critics of that program have tried _ unsuccessfully _ to stop the practice through lobbying of government agencies and appeals to Congress.

Now they're asking a U.S. District Court judge to intervene. The lawsuit filed Monday would bar the National Park Service and Forest Service from participation in the slaughter program.

The plaintiffs contend the two federal agencies have become beholden to the cattle industry and are ignoring their wildlife conservation mission. They also say the threat of the disease, brucellosis, has been overstated.

"It's crazy for me to think that in a state like Montana, where we are rich in wildlife and wildlands, that we don't have room for bison," said Tom Woodbury with the Western Watersheds Project.

Woodbury's group is one of nine plaintiffs in the case, which will likely be heard by Judge Donald Molloy in Missoula.

Bison once roamed North America by the millions before being largely wiped out in the late 1800s. Yellowstone's 3,000 bison comprise one of the largest concentrations of the animals remaining in the world.

About half those animals test positive for exposure to brucellosis, a reproductive disease that causes pregnant animals including cattle to abort their young.

During severe winters and when bison numbers are high, thousands of bison attempt to migrate to feeding grounds outside Yellowstone. But under a 2000 agreement between Montana and the federal government, the animals can be killed to prevent any contact with cattle.

In 2008, more than 1,400 bison were captured and shipped to slaughter under that policy, cutting the park's population by more than a third.

Soon after, the Government Accountability Office released a scathing report admonishing Montana and federal agencies for failing to preserve Yellowstone's bison.

As a result, government agencies promised to be more flexible. They allowed bison to leave the park for some areas where cattle no longer graze. And a migration corridor was leased through a private ranch adjacent to Yellowstone to let at least 25 bison access Forest Service land outside the park.

Representatives of the cattle industry fought both moves. Continued...