Wildlife experts have no explanation for why a grizzly sow and her three cubs rampaged through a campground outside Yellowstone National Park late last month, attacking two campers before killing and partially consuming a third.

A 70-page report released Monday portrayed a sow intent on killing humans in a campground that, while in the heart of grizzly territory, had not encountered any bear problems this year and which had been properly maintained and kept clean so as not to lure bears.

While a medical analysis (necropsy) of the sow showed her to be somewhat light in weight and infested with parasites, that alone did not explain why she led her cubs on a predatory attack on the Soda Butte Campground in the Gallatin National Forest just beyond Yellowstone's northeast entrance.

"There is no clear explanation for the aggressive, predatory behavior of this adult female grizzly bear in the early morning hours of 28 July 2010," concluded the investigative team, which included representatives from state and federal wildlife agencies and also included Dr. Stephen Herrero, a University of Calgary professor who long has studied the behavior of black and grizzly bears. "The bear and her offspring were low in body condition but their body condition was not outside the range of other wild grizzly bears in the Yellowstone ecosystem."

For campers heading into bear territory in the weeks and months ahead, the report reads ominously, describing a scene where campers evidently did everything they should when staying in grizzly habitat and yet still fell victim.

The sow presumably led her three yearlings through the Soda Butte Campground in search of food. Within a period of perhaps two hours they attacked three tents, were chased or scared off from two, and then succeeded in killing a camper who had pitched his tent about 180 feet from anyone else -- and approximately 600 yards from where the first two campers were attacked.

In the early morning hours of 28 July 2010, an adult female grizzly bear accompanied by 3 yearlings attacked 3 separate people in 3 different tents in the Soda Butte Campground. The initial attack was inflicted on Mr. Ronald Singer at approximately 0200 hours, who was bitten through his tent on his lower left leg. Mr. Singer punched the bear several times and the bear left. The second attack was inflicted on Mrs. Deborah Freele at approximately 0215 hours; she was initially bitten on her upper left arm and then bitten on her lower left arm. She then received a slight bite to her left leg and then the bear left. The third attack was inflicted on Mr. Kevin Kammer at an unknown time, presumably after the first 2 attacks. Mr. Kammer was camping by himself, and was killed and partially consumed at his campsite. All of these attacks occurred in the 27-site campground, of which 24 sites were occupied by people on the night of the attacks.

While the 15-page report, supported by another 55 pages of reports and interviews, raised possible motivations for the bears' behavior, the authors did not pinpoint any as a cause for the attacks. The 216-pound sow had apparently not been conditioned to connect humans with food; analysis of her hairs indicated that her diet had been almost exclusively vegetative for the past two years, and; campers were careful with food storage -- "there was no evidence of improper food storage problems anywhere in the campground at the time of the attacks or prior to the attacks, nor was there evidence that bears obtained garbage or human foods in the campground in 2010."

And yet, the bears seemed intent to find human prey when they entered the campground, which is set amid stands of spruce and pines, and moved from tent to tent to tent before succeeding in killing Mr. Kammer, a 48-year-old Grand Rapids, Michigan, man. The man's body, which had been pulled through a hole in his tent and then dragged about 10 yards from his tent, was found shortly before 4:30 a.m. by law enforcement officers who were clearing campers out of the campground.

In all three incidents, the bears had bit or ripped through the tent fabric to reach the campers, according to the report. While it was not clear whether the sow alone killed Mr. Kammer, the investigators said the evidence indicated that the yearlings partially consumed the man.

Bears had been visible in the immediate area in the days leading up to the attack, according to the report. A subadult grizzly, which investigators determined was not involved in the attacks, had been photographed three days earlier; paw prints of an undetermined bear species had been seen on some of the campground's bear boxes and garbage bins, and a week before the attack a jogger spotted a grizzly sow with three yearlings about 9 a.m. between Yellowstone's entrance and the hamlet of Silver Gate.

"A grizzly bear with 3 yearlings came out of the woods and onto the highway," the report noted. "The mother stood up, then charged Elaine Sabo. She screamed at the bear, 'Hey, hey, hey,' and the bear stopped. The bear and her yearlings then ran into the woods and Ms. Sabo returned to the northeast entrance station. This sighting is approximately 4-5 air miles west of Soda Butte Campground."

Interestingly, DNA analysis of hairs collected by investigators indicated that a fifth grizzly had been in the campground, though not necessarily during the attacks. The hair was found in one of Mrs. Freele's shoes, and the investigators guessed that she might have picked it up sometime during the previous 13 days that she had camped there.