Authorities said three separate attacks left a male dead and a woman and another male injured at the Soda Butte campground. The woman suffered severe lacerations and crushed bones from bites on her arms, and the surviving male was bitten on his calf.
Wildlife officials did not release the names or ages of the victims, but said they were in three different tents.
Don and Paige Wilhelm of Aledo, Texas, were spending the night with their two boys in the campsite next to the woman when they first heard a scream. It was coming from several sites away where, they later discovered, a boy was being attacked.
"We weren't sure what it was. We thought maybe teenagers yelling," Don Wilhelm said. They tried to go back to sleep, but 10 minutes later the bear was tearing into the tent of the woman, who they said was from Canada and about 50 years old.
"First she said, "No!' Then we heard her say, 'It's a bear! I've been attacked by a bear!" Paige Wilhelm said. As the animal snorted and huffed outside their tent, the Wilhelms first thought they would lie on top of their children to protect them from what seemed an inevitable attack. But after the noises outside stopped, they bolted for their SUV.
Don Wilhelm aimed the headlights at the woman's campsite, "and we could see her there, kind of half in her sleeping bag. I don't remember seeing any tent," he said.
With their 9- and 12-year-old boys pleading for them to stay in the vehicle, the Wilhelms decided to drive through the Soda Butte Campground, honking their horn and yelling at other campers to alert them.
At one point they encountered a truck leaving the campground. Inside was the third victim - a teenager who apparently attempted to fight off the bear by punching it in the nose and face after it entered his family's tent and bit into his leg.
Don Wilhelm said he later returned to the Canadian woman's campsite with two other men and helped a nurse staying at the campground bandage her wounds. Wilhelm says they didn't find out a man had been killed in the attack until later Wednesday.
Both survivors were taken into nearby Cooke City - a tourist town just outside Yellowstone - where an ambulance later picked them up and took them to a hospital in Cody, Wyo.
The victims were in three different tents, according to Montana Fish, Wildlife and Parks Warden Capt. Sam Sheppard. Two of the tents were close to each other. The male who was killed was alone in a tent about a quarter-mile away in the heavily occupied campground that has 27 sites for tents and recreational vehicles, Sheppard said.
All three victims had their food in storage boxes, Sheppard said.
"They were doing things right," Sheppard said. "It was random. I have no idea why this bear picked these three tents out of all the tents there."
Wildlife officials were inspecting the campground, which is run by the U.S. Forest Service, to determine what happened.
"We don't know if it was one bear, two bears, a black bear or grizzly bear," Fish, Wildlife and Parks spokesman Ron Aasheim said. "Obviously, the bear's gone now. Will it come back tonight? That's the question."
Authorities set five baited traps and were collecting bear hair, saliva and droppings while measuring the victims' bite wounds to determine the type and number of bears involved. Two of the traps were set where the male who died was camping.
The campground was closed, but from the roadside a large hole could be seen in the side of a small, brown tent still set up at the site.
If the bear is caught, it will be killed, Sheppard said.
Park County dispatchers took a 911 call early Wednesday from a male reporting that a bear had bitten his ankle and was tearing up tents, Aasheim said. Dispatchers got two more calls, including one from a man who said a bear bit the leg of his daughter's boyfriend.
At 3:50 a.m., park officials went through the campground to advise campers to get into their cars. A half-hour later, the dead male was discovered at a campsite. Authorities evacuated the campground, sending campers to nearby hotels.
It was not immediately clear how many people were in the campground at the time.
The same campground was the site of a 2008 attack in which a grizzly bear bit and injured a man sleeping in a tent. A young adult female grizzly was captured in a trap four days later and transported to a bear research center at Washington State University in Pullman.
"Now the suspicion among a lot of the residents is that the bear they caught (in 2008) was not the right one," said Gary Vincelette, who has a cabin in nearby Silver Gate.
Last year, another grizzly broke into three cabins in the nearby community of Silver Gate, said Vincelette. That bear was shot and killed by a Silver Gate resident when it returned to the area.
"Three attacks in three years - we haven't ever had anything like that, and I've been coming up here since I was a kid," Vincelette said.
The 10-acre Soda Butte campground is in Gallatin National Forest, just off the mountainous Beartooth Highway about 125 miles southwest of Billings.
"It is a populated area for bears, not just grizzly bears but black bears," Gallatin National Forest spokeswoman Marna Daley said.
Two other nearby campgrounds also have been closed, Daley said. Forest Service officials will consider shutting down more campgrounds after consulting with state wildlife officials leading the investigation, she said.
AP Writers Matt Volz and Amy Beth Hanson in Helena contributed to this report.