Yellowstone National Park to CLOSE!

The fatal mauling occurred about a mile and a half from the start of the Wapiti Lake trail, and another group of hikers nearby heard the victim's wife crying out for help and used a cell phone to call park rangers for assistance.

A National Park Service statement said the couple had inadvertently surprised the mother grizzly and her cubs, and in "an attempt to defend a perceived threat to her cubs, the bear attacked and fatally wounded the man."

The victim's identity and hometown have been withheld pending notification of other family members.

The bears involved in Wednesday's encounter remained at large for the time being.

Authorities are still investigating the circumstances of the attack, but initial information indicated the mother bear behaved normally in defending her cubs and would not be killed as a result of her actions, park spokeswoman Linda Miller said.

Attacks by bears are extremely rare. No visitors were injured by bears in Yellowstone during all of last year, and Wednesday's incident marked the first bear-caused human death in the park since 1986, the Park Service said.

But a mother grizzly killed one man and injured two other people in an unusual night-time attack on sleeping campers just outside Yellowstone in Montana last July. The bruin involved in that incident was later trapped and destroyed because the attacks were considered to be unprovoked.

A bear warning sign is posted at Yellowstone's Wapiti Lake trailhead because it is an access point to the Pelican Valley area known for significant bear activity. No bear encounters had been reported along or near that trail this season, the Park Service said.