YELLOWSTONE NATIONAL PARK, Wyo. - A grizzly bear that mauled a 57-year-old hiker to death in Yellowstone National Park was only defending its cubs and had not threatened humans before. So park officials on Thursday decided to leave it alone to wander the backcountry.

The mauling - the park's first in 25 years - temporarily closed one of the park's top attractions, the south rim of the Grand Canyon of the Yellowstone, on one of the busiest days of the year. Some tourists were left to wonder what was going on.

"It was not predatory and so we see no reason to take action against the bear," said Kerry Gunther, bear management biologist for Yellowstone.

The attack also highlighted the potential dangers, however rare, that face tourists who come in record numbers each year to a park known for its burgeoning bear population and the Old Faithful geyser.

Whenever there is a run-in or attack involving bears, park officials must decide whether the attack was defensive or an act of aggression. In Wednesday's mauling, they based their conclusion on the account of the hiker's wife, who survived, as well as their knowledge of bear behavior.

Brian and Marylyn Matayoshi, of Torrance, Calif., were hiking in a backcountry meadow along a trail a mile and a half from the trailhead when they spotted the bear foraging about 100 yards away. The couple immediately turned and began walking away, officials said.

The grizzly charged and attacked Brian Matayoshi, then went for his wife, who ran for cover behind a tree. The grizzly lifted her off the ground by the day pack she was wearing and then dropped her.

She tried to call 911 on her cell phone, but couldn't get a signal. Other hikers in the area responded to her cries for help and managed to get through to emergency officials.

Marylyn Matayoshi told rescuers that the couple surprised the sow, its cubs nearby - one of the most dangerous situations possible for humans encountering grizzlies. Park officials believe the grizzly had two six-month-old cubs, based on previous sightings in the area and cub tracks where the attack occurred.

"All indications are that this was a defensive attack," park spokesman Al Nash said. "In such cases, the park's policy is to leave the bear in the backcountry."

The bear had never been documented before, never been tagged, and there was no reason to believe it had interacted with humans before, Nash said.

They said the way the attack happened indicated the bear didn't intend to eat the couple.

Marylyn Matayoshi escaped injury and was no longer at the park, and officials declined to reveal her whereabouts.

In Torrance, neighbor Kathy Hester said Matayoshi and his wife kept their house immaculate and recently had put in a new lawn. "They are the sweetest people you'd ever want to meet," Hester said.

Park officials called the mauling a "1-in-3-million" encounter.

While many visitors Thursday morning were unaware of the attack, many seemed to know about it by the afternoon. Desk clerks at hotels inside the park told new arrivals that there had been a bear mauling. Worried relatives called or texted other visitors.

Some were surprised that rangers didn't let them know when they entered the park that there had been an attack and that some trails were closed.

"They didn't say one word about it at the gate," said Leslie Finch, visiting with her husband and two children for two days from Missoula, Mont. "I would have thought they'd say this area is closed. But they didn't say anything."

Park officials said the attack shouldn't condition the sow to attack again. They also collected DNA samples from fur at the attack site, so they can determine if the bear is involved in another attack, Gunther said.

"We don't believe that this defensive action by the bear would make any future action more probable," park superintendent Dan Wenk said.

Decades of research has established that grizzlies, while dangerous, rarely get aggressive with people except under very predictable circumstances, said Mark Bruscino, a Wyoming state bear biologist who has investigated some 40 attacks.

Grizzlies become aggressive when they are harassed, taken by surprise up close, are defending a food source or are defending their cubs, Bruscino said.

"You can almost explain every incident that occurs with a grizzly bear around those four," he said.

Bruscino declined to weigh in on the decision not to track and kill the Yellowstone bear.

A bear that fatally attacked a man and seriously injured two people at a campground east of Yellowstone last July was killed in part because the circumstances didn't neatly fit into predictable bear behavior, he said.

Hunger and internal parasites afflicted that grizzly, but investigators said they could not explain its late-night rampage through the crowded campground near Cooke City, Mont. That grizzly was captured and euthanized. Its three cubs were taken to a Billings, Mont., zoo and were recently moved out of state.

Wednesday's mauling was the park's first fatal grizzly attack since 1986, but the third in the region in just over a year amid ever-growing numbers of grizzlies and tourists roaming the same wild landscape. In June 2010, a grizzly just released after being tranquilized for study killed an Illinois hiker outside the park.

Grizzlies are an omnivorous species with a diet of berries, elk, fish, moths, ants and even pine nuts. Officials routinely urge visitors to take precautions: Stay on designated trails, carry bear spray, hike in groups of three or more, and make noise in places where a grizzly could be lurking.

The decision not to track and kill the Yellowstone bear isn't unprecedented. In nearby Grand Teton National Park, officials decided not to intervene with a grizzly that wounded a man in 2007. Rangers determined that female also was defending its cubs and didn't pose a general threat to humans.

"This is bear country," said Elizabeth Hoffman, a tourist from California who agreed with park officials' decision. "It's got babies. If someone came after a human mother, I don't think that we'd take her from her children."

Associated Press writers Mead Gruver in Cheyenne, Wyo., and Greg Risling in Los Angeles contributed to this report.