A husband and wife's backcountry hike along a popular trail turned tragic when they stumbled upon a grizzly bear and her cubs and the 57-year-old man was mauled to death, Yellowstone National Park officials said.
The couple was hiking along the Wapiti Lake Trail in the Grand Canyon area of the park, park officials told ABC News. They had walked about a mile and a half from the trail head when they saw the grizzly sow and her cubs.
"The bear attacked the man and killed him," said Yellowstone spokesman Al Nash.
The woman screamed out for help as the bear was attacking her husband. Nearby hikers heard the pleas and dialed 911. By the time park rangers arrived, the man was dead.
"The initial indication is the sow grizzly was protecting her cubs," Nash said. "The investigation, as it unfolds, will help us determine if that, indeed, was the case."
The park is not releasing the identities or the hometown of the couple.
The wife suffered bumps and bruises and did not require medical attention, officials said.
After the attack, all of Yellowstone's backcountry campsites and hiking trails in the area were closed. Rangers were on patrols to clear out all visitors from those backcountry areas, park officials said. Bear warning signs have been posted along the trail.
There had been no reports of bear encounters in the area this season. There also had been no reports of animal carcasses found, which often can be a sign of bear activity.
Rangers are looking for the bear, Nash told ABC News.
Currently, however, there is no effort to trap the bear, officials said, because indications are it was a defensive attack, the result of the sow protecting her cubs.
Today's incident is the first bear-caused human death in the Yellowstone park limits since 1986.
"It is extremely unfortunate that this couple's trip into the Yellowstone backcountry has ended in tragedy," Yellowstone National Park Superintendent Dan Wenk said.
Yellowstone's hiking trails are typically packed with visitors this time of year. Bear encounters are rare but they do happen.
Those who decide to hike on Yellowstone's trails, officials said, should hike in groups of three or more people, make lots of noise and carry bear pepper spray. It was unknown if the couple attacked was carrying bear pepper spray.
ABC News' Clayton Sandell contributed to this report.