SALMON, Idaho (Reuters) - Authorities were investigating whether a grizzly bear found dead outside Yellowstone National Park was killed illegally, they said on Tuesday.

An examination of the site, a few miles south of the park, and the decomposing carcass, show the bear did not die of natural causes, U.S. Fish and Wildlife Service officials said in a statement.

They declined to provide additional details about the dead bear, which was recently discovered by hunters.

The estimated 600 grizzlies in the Yellowstone National Park region, which spans parts of Wyoming, Idaho and Montana, are protected under the U.S. Endangered Species Act.

That generally bans people from killing the bears unless they are threatening human life. In those cases, authorities are to be notified.

Gregg Losinski, regional conservation educator for the Idaho Department of Fish and Game, said it is relatively rare for a grizzly to fall prey to "a mysterious death" in the state.

Fewer than three dozen of those bears roam the rugged backcountry in eastern Idaho on the Wyoming border where the carcass was found.

(Reporting by Laura Zuckerman; Editing by Dan Whitcomb and Greg McCune)