Three hunters charged by a grizzly bear in Grand Teton National Park last Thanksgiving Day acted in self-defense in shooting and killing the bear, an investigation has determined.

After reviewing the investigation, handled by law-enforcement rangers at the park in consultation with the U.S. Fish and Wildlife Service, the U.S. Attorney's Office decided not to file any charges against the three, park officials announced Thursday.

According to the investigation, the unidentified hunters, who were participating in the park's annual elk reduction hunt, tried to deter the bear with bear spray, but when it charged to within 10 feet of the trio two fired at it, hitting the grizzly twice in the head and once in the back, killing it, the investigation noted.

The hunters immediately reported the matter to park authorities and fully cooperated with the ensuing investigation, which concluded that the overall encounter lasted less than 10 seconds, a park release noted.

"During that brief time, the hunters deployed bear spray and discharged firearms against the charging grizzly. Park rangers and science and resource management personnel believe that both the bear spray and bullets contacted the grizzly bear at nearly the same instant," the release said. "The totality of circumstances indicated that the hunters were forced to make rapid decisions in close proximity to the bear, and they acted in self-defense."

According to the investigation, at 7:25 a.m. on November 22, 2012, two Grand Teton rangers were on routine patrol, making hunter contacts at Teton Point Overlook, when they heard five gun shots in less than 5 seconds; the first two shots were followed in rapid succession by three more. At 7:32 a.m., a woman called the Teton Interagency Dispatch Center to report that her husband and sons had been charged by a grizzly bear and they shot at the animal, the report went on.

Park law enforcement rangers and wildlife biologists responded and began an investigation into the incident. Rangers met with the hunting party and all three men fully cooperated with the investigation. The three hunters (ages 48, 20 and 17), all from Wyoming, had permits to participate in the elk reduction program at Grand Teton National Park. All three carried bear spray as required for this wildlife management program.

According to interviews, the hunting party left the parking area at Schwabacher Landing at first light and had just entered into a timbered area in the Snake River bottom, slightly north of Schwabacher Landing and west of Teton Point Overlook, when the oldest of the group first noticed the bear. Although he tried to scare the bear off, it began to charge the group from 42 yards away, the investigation noted.

"One member of the group described the grizzly bear as moving 'like a cat,' incredibly fast, snapping tree branches, and moving very low to the ground," the investigators noted.

All three hunters had bear spray readily accessible. The oldest member of the group immediately began deploying his bear spray while the two younger hunters raised their rifles. When the grizzly bear came within 10 feet of the young men, they both fired shots.

During the investigation, a partially consumed and cached elk carcass was discovered 50 yards away, leading park biologists to conclude that the bear was defending its food source. The fatally injured male bear weighed 534 pounds and was estimated to be 18 to 20 years old.

Grand Teton National Park managers expressed regret over the loss of the grizzly, but noted that the hunters involved in the incident made sound decisions after their bear encounter ended.

This is believed to be the first instance since the elk reduction program began in 1950 that a grizzly bear has been killed by hunters in Grand Teton National Park. The largest source of known grizzly bear mortalities in Grand Teton have actually resulted from vehicle collisions, with a total of five grizzlies killed on park roads during 2005-2012 alone.

To date, encounters between humans and grizzly bears that resulted in injuries to people are relatively uncommon. However, during the last 20 years as the Yellowstone ecosystem grizzly bear population has recovered and regained formerly occupied habitat (including in Grand Teton National Park) bear maulings have increased. Grand Teton has documented six attacks since 1994, when a jogger was mauled on the Emma Matilda Lake trail. Other maulings occurred in 2001, 2007 and 2011. A mauling also occurred in the John D. Rockefeller, Jr. Memorial Parkway in 1997. None of these bear attacks resulted in fatal injuries to humans.

Although possessing and carrying firearms in national parks is legal, the “use” of firearms is still prohibited under 36 CFR 2.4 (a)(1)(iii), unless permitted for specific purposes such as the elk reduction program. As a condition of their participation in the elk reduction program, hunters are only permitted to shoot an elk.

In light of this incident involving the fatal shooting of a grizzly bear, park managers are reviewing steps that might be taken to reduce such incidents in the future.