Tents were smashed in the 4 a.m. attack that left a male dead at the Soda Butte campground. A female suffered severe lacerations from bites on her arms, while another male was bitten on his calf and taken to a hospital in Cody, Wyo.
Wildlife officials did not release the identities or ages of the victims. A response team was being sent to piece together what happened.
"We don't know if it was one bear, two bears, a black bear or grizzly bear," Montana Fish, Wildlife and Parks spokesman Ron Aasheim said. "Obviously, the bear's gone now. Will it come back tonight? That's the question."
Authorities will set traps and seek bear hair, saliva and droppings while measuring the bite wounds of victims to determine the type and number of bears involved in the attack.
Authorities cleared the campground after the attack was reported at 6 a.m., telling campers to go to their cars.
It was not immediately clear how many people were in the campground at the time of the attack.
A Cooke City resident who chose to remain anonymous told CBS affiliate KTVQ that there were two separate attacks, both causing severe injuries. The source says there may have been a third attack.
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 after the attack and transported to a bear research center at Washington State University in Pullman.
The 10-acre Soda Butte campground has 27 sites for tents and recreational vehicles in the Gallatin National Forest, some five miles from the northeastern entrance of Yellowstone National Park.
It is located just off the mountainous Beartooth Highway about 125 miles southwest of Billings.
The dead male was found at a campsite at the western edge of the sprawling campground. It was not immediately clear where the other two victims were located.
"It is a populated area for bears, not just grizzly bears but black bears," Gallatin National Forest spokeswoman Marna Daley said.
The campground, which is run by the U.S. Forest Service, has been closed, as well as two other nearby campgrounds, Daley said. Forest Service officials will consider closing more campgrounds after consulting with state wildlife officials leading the investigation, she said.