This 2013 photo shows a grizzly bear at Mary Bay along Yellowstone Lake in Yellowstone National Park. (Marc Cooke/Wolves of the Rockies/AP)

Park officials may never know for sure what happened between Lance Crosby and the bear that attacked him in the backwoods of Yellowstone. There were no witnesses to the incident that left the 63-year-old veteran hiker dead amid pine needles and dirt half a mile from the nearest trail, so it’s unclear what could have pushed a normally reclusive mother grizzly to maul him and leave his body “partially consumed.”

[Grizzly suspected of killing Yellowstone hiker will likely be euthanized with cubs]

The attack was a tragedy for Crosby’s family and friends, and for the park, where Crosby had long worked as a summer employee.

It also seems likely to end sadly for the bears. Park officials have captured the sow believed to have killed Crosby along with one of her two cubs. If DNA evidence matches her to the killing, she will be euthanized and her cubs offered to a rehabilitation center or zoo. If they can’t be placed, the baby bears — members of a federally protected threatened species — will also be put down.

For park officials who have dedicated their lives to the protection of wilderness, cases like this are painful and deeply unsettling. On the one hand, they have 3.5 million yearly park visitors to protect. On the other, all 3.5 million of those visitors, along with park employees themselves, are interlopers on the bears’ land.

How can we condemn animals for their wild behavior?

Answering that question involves careful calculus and a forensic investigation worthy of its own procedural drama. Call it “CSI: National Parks.” At the end of the day, though, the bear almost always pays for a human life with its own.

Grizzlies are the most feared of North America’s bears, helped in no part by their name: Ursus arctos horribilis, or “horrible bear.” They have a reputation for aggression that wildlife conservationist Richard H. Yahner says is partly due to their evolution. Unlike the more docile black bears in the East, grizzlies have trouble climbing and give birth to just a handful of offspring. That means that grizzlies, particularly mothers defending their young, have learned to stand their ground.

Even so, the bears have caused relatively few fatalities in Yellowstone’s 143-year history. Despite their obvious physical advantage — adult grizzlies weigh several hundred pounds and boast jaws that can crush a bowling ball — the bears don’t consider humans to be prey. Visitors to Yellowstone are about as likely to be killed by a lightning strike as a bear attack.

Indeed, for many years it was the bears who had reason to fear humans. A combination of habitat loss and hunting pushed Yellowstone’s grizzly population to the brink of vanishing. In 1975, just before the species was assigned threatened status by the U.S. Fish and Wildlife Service, there were only 136 bears left in the park, and about 550 in the entire continental U.S.

In 1986, a group of wildlife biologists called the Interagency Grizzly Bear Committee put out a set of guidelines for grizzly management — addressing, among other issues, what to do if a bear poses problems for humans. The committee concluded that unless the animal in question is deemed a “nuisance,” grizzly-human conflicts must be resolved in favor of the bear. “Nuisance” bears must meet at least one of three conditions: (a) they have been raiding livestock or food stores that have been reasonably well-protected, resulting in their “conditioning” to seek food from people; (b) they act aggressively in a way that suggests an immediate threat or (c) they cause significant injury or loss of life in an unprovoked, non-defensive encounter with humans.

In other words, bears that act like bears shouldn’t be punished for doing so. In most cases, they are simply responding to human provocation in the only way they know how, and the attack is a one-off incident. But bears that behave strangely are another matter. If they launch an unprovoked attack once, who’s to say they won’t do it again?

In the years after the guidelines were released, officials at Yellowstone rarely had reason to reference them. By 2010, the park hadn’t seen a fatal bear attack in nearly a quarter of a century.

But Yellowstone’s bear population was surging back to normal levels — there are now around 700 of them within the park — and visitor numbers were soaring. More bears and more humans meant that the two groups were increasingly coming into contact, with increasingly deadly consequences.

In July 2010 two people were injured and a third killed, his body partially eaten, in an attack on a campground just a few miles from the park’s northeastern entrance. Another man hiking just outside the park’s eastern boundary was mauled and killed by grizzly that had just been tranquilized, fitted with a collar and released by researchers.

The following year saw two more fatal attacks within the park’s borders.

Brian Matayoshi was hiking with his wife in early July when he came upon a mother with two cubs and started running and yelling, according to the New York Times. The mother bear, known as the Wapiti sow, charged after them. She knocked the man down and bit his leg several times. His wife, Marylyn, lay motionless, feigning death. The sow picked her up by her daypack, then dropped her. When it finally left, Marylyn scrambled to her husband’s aid and attempted to use her jacket as a tourniquet, but he was already dead.

The next month, experienced hiker John Wallace ventured up a trail just eight miles from where Brian Matayoshi had been killed. According to Outside, Wallace had declined the standard bear-safety literature distributed by Yellowstone, telling rangers that he was “a grizzly bear expert.” Two days later, his remains were found on the trail, his open daypack lying nearby. Wallace’s arm and hand showed bite marks indicating he had tried to defend himself — the same kind seen on Crosby.

In each of these cases, wildlife officials had to refer back to IGBC’s guidelines. Had the bear been conditioned to steal food from humans? Was the fatal incident defensive, or provoked?

The brazen campground attack in 2010 was clearly out of the ordinary; the sow responsible was trapped and killed, and its two cubs given to a zoo.

The circumstances surrounding the attack by the collared bear were less clear. The hiker involved, reportedly an experienced outdoorsman, had bypassed warning signs about a bear in the area. Even his friends said that he suffered from a combination of bad luck and bad judgment. Could the bear be blamed for the hiker’s mistake?

Ultimately, wildlife officials — who faced a lawsuit from the hiker’s family alleging that they’d failed to provide adequate warning about the risk — opted to kill the bear, since they couldn’t say for sure whether the encounter had been the human’s fault.

“We try to do everything we can to minimize the risks. But we can’t protect ourselves against people that ignore every warning we give, and we can’t protect people against themselves,” grizzly bear coordinator for the U.S. Fish and Wildlife Service Chris Servheen told the Billings Gazette. “The whole thing is regrettable; just one tragedy followed by another.”

The 2011 attacks were likewise difficult to classify. The Matayoshis had done everything you’re not supposed to do in a grizzly confrontation — they ran, they yelled, they failed to carry pepper spray (which has been proven to deter 90 percent of attacks). And the sow seemed to be defending her two cubs, common mother bear behavior. Initially, the sow was allowed to go free.

But the discovery of Wallace’s mangled remains later that summer launched a huge investigation into the bear behind his death. The body had been buried under debris, or “cached” — how grizzlies hide their food. This case fell into the second category of human-bear encounter, the one whose penalty was death.

According to Slate, park investigators gathered DNA evidence from the scene and identified the bloody pawprints of a mother and her cub not too far away. Lab analysis pointed to the Wapiti sow, the same mother grizzly who had killed Brian Matayoshi. Traps were set, and for a month the rangers watched and waited with intensifying dread. When the bear and her cubs were finally captured, hair and blood samples matched those taken from both crime scenes. The Wapiti sow would have to be euthanized.

It wasn’t an easy choice — though Yellowstone’s grizzly population has rebounded, its success relies on the fact that 91 percent of bears make it through each winter. There were only 250 reproductive females like the Wapiti sow in the park in 2012.

But park officials serve their human patrons, not their ursine charges. Sometimes a bear has to be killed in order to “err on the side of human safety,” Yellowstone wildlife biologist Kerry Gunther told Slate in 2012.

Still many park officials believe that the increased presence of humans, particularly humans who are uneducated or careless about dealing with grizzlies, is sparking more of these deadly confrontations. Not because bears get a “taste for human blood” — speaking to Slate, Servheen dismissed that idea as something from “horror stories in movies” — but because humans who get too close to bears can trigger defensive responses, and bears that are too accustomed to humans are more likely to launch an un-bear-like predatory attack.

It’s better for humans to stay wary of bears and bears to be fearful of humans, officials say. Both our lives and the bears’ lives might depend on it.