A grizzly bear shot dead while charging hunters in Grand Teton National Park last Thanksgiving closed in from 50 feet away in fewer than 10 seconds, the three told investigators.

Investigators determined the shooting to be self-defense. Federal prosecutors will not file charges against the two young hunters — 17 and 20 years old — who fatally shot the bear, park officials said Thursday.

It was the first grizzly killed in connection to the park’s 63-year-old elk reduction hunt.

“By all independent accounts [it was] a very fast moving and dynamic event where the individuals had only seconds to react.” Grand Teton ranger and lead investigator Jim Dahlstrom wrote in a report.

The bear was defending its food, the report suggests. Investigators found a partly devoured cow elk 42 feet from where the grizzly died. Vegetation near the cow’s carcass was trampled, and investigators believed the bear was bedding down there.

A “case incident report” released by the park included statements from all three hunters and dozens of photographs.

The party members’ names were blacked out, but the News&Guide identified them as Dubois resident David Trembly and his two sons.

The Tremblys have repeatedly refused interviews with the News&Guide.

All three hunters were carrying bear spray, a requirement in the park, and had licenses, the report said. David Trembly had filled his tag and wasn’t armed.

Hunting near the Snake River northeast of Schwabacher Landing, the family moved into thick timber to “stay out of the fireline,” David Trembly said in a written statement submitted to investigators.

At about 7:25 a.m., he spotted a moving “horizontal line” in downed timber about 25 yards away.

“The bear’s head came up,” Trembly said. “I stood tall and yelled ‘go bear, no bear.’”

At that point, Trembly said, the grizzly, a 534-pound, 18- to 20-year-old male, “immediately started at us.”

Trembly blasted a cloud from his bear spray canister, but the bear continued “moving incredibly fast, snapping branches and moving very low to the ground, like a cat,” he told Dahlstrom.

“It was not a bluff charge,” Trembly said. “I was sure it was going to get [name redacted].”

At 30 feet, Trembly said he tried to “direct spray” the grizzly, but trees got in the way.

“I moved to try to get in between the bear and [names redacted], but the bear was to [sic] fast,” Trembly said.

The bruin continued closing in and was “not phased [sic].” His sons shot when Trembly was still spraying, he said, and he continued spraying after the bear was down.

“A confounding issue with this particular canister of bear spray include it being expired for more than 9 years, which may have compromised the propellant, active ingredients or both,” the report said.

“The bear was 15 feet from us running directly at us,” one of Trembly’s sons said in a statement. “I shot and immediately after [name redacted] shot [the] bear dropped 11 feet from our feet.”

Once the bear was on the ground, the hunter shot two more rounds into its head.

“It was still moving, and I thought it was going to get up,” one of the sons told Dahlstrom.

Once the bear stopped moving, the Trembly family dropped their packs and bear spray in place and hiked out. They returned with investigators to re-enact the encounter, photographs from the report show.

All party members’ stories were consistent, Dalhstrom found.

Orange residue from Trembly’s spray was “clearly visible” 20 feet away from the dead bear on the ground and on three trees, his report said. Bear spray was also found on the animal’s snout.

“It is most likely that the bear came into full contact with the bear spray just before it was killed,” Dahlstrom’s report said.

Other evidence corroborated the Trembly’s story.

Two park rangers heard five shots fired in rapid succession “minutes before” receiving the initial call.

“Freshly broken branches” were found between the elk carcass and the bear’s carcass.

“The path behind the bear indicates that it was charging, and when one of the bullets struck either the head or the spine causing it to fall to the ground, its forward momentum may have created the ‘skid marks,’ observed on the forest floor,” the report’s conclusion said.

“After reviewing all evidence, oral and written statements, and conferring with all involved [National Park Service] law enforcement and [Wyoming] Game and Fish personnel, it appears as though the group’s actions were in self defense.”