A female grizzly bear attacked and killed a man who encountered the bear and her cubs while he was hiking with his wife on Wednesday in Yellowstone National Park, park officials said.
The attack happened about a mile and a half from the start of the popular Wapiti Lake trail and after hearing a woman's screaming for help, a group of hikers who were nearby called the park rangers.
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".
Grizzly bears can weigh up to 1,500lb and despite their size, they can run at speeds of up to 35mph, according to the US Fish and Wildlife Service.
The victim's identity and hometown have been withheld pending notification of other family members.
The bears involved in Wednesday's encounter have not been captured, and park officials said they did not immediately have enough information to determine what measures, if any, they might take in the aftermath of the attack.
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.
However, bears found to have had repeated run-ins with park visitors are sometimes relocated.
"If we have an aggressive bear that continually poses a threat to human safety, then we work to remove it from the ecosystem," Yellowstone spokesman Al Nash said, adding he did not know if this grizzly had any previous human encounters.
Park officials have focused their initial response to Wednesday's mauling on closing the Wapiti Lake trailhead and removing any remaining hikers from the area.
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.