"There was an earlier contact with the bear prior to the event," Powell County Sheriff Gavin Roselles told CBS News. "The bear basically came back into the campsite. It wandered into a campsite a couple different times."
While Grizzly sightings are common in the area, attacks are not.
"This isn't normal bear behavior, and it's the kind we want to address right away," Montana Department of Fish, Wildlife & Parks spokesman Greg Lemon told the Independent Record.
"Most of the time when we have grizzly-human encounters that result in injury, most of the time the bear is doing normal bear stuff — protecting food resources, protecting cubs or a surprise encounter. This doesn't really apply in this situation, where somebody was camping at night."
"We have a bear roaming around town, that had already hit a chicken coop, so our response is different than if it was normal bear behavior."
Grizzlies involved in non-fatal attacks on humans are frequently spared, if it was a mother protecting her cubs, or a surprise encounter.
But according to authorities a bear that kills must also be killed — if they can find it.
The grizzly left the campsite after somebody deployed bear mace. Wildlife specialists are now combing the area on foot and by helicopter, and setting traps in the hopes of catching and destroying the animal.
"Everybody's pretty shaken up right now. The population here is 75 - everybody knows everybody," local saloon owner Tiffanie Zavarelli said. "The people from Montana, we know how to be 'bear aware.' But anything can happen."
An adult grizzly can weigh up to 1,500lbs and can stand anywhere between 6ft and 10ft tall on its hind legs. They are a protected species, classified as threatened, and therefore cannot be hunted.