In 1976, Gary Frank Sotherden’s appetite for adventure and the outdoors led him to the Arctic Circle, where he and a friend planned to walk on opposite sides of the Porcupine River in northeastern Alaska, reuniting when the cold set in and the river froze, his brother said.

The friend made it out, but Mr. Sotherden was never heard from again.

Planes flew above the remote, winding river in search of Mr. Sotherden, who was from Clay, N.Y., which is about a dozen miles northwest of Syracuse. The police and mountain guides also searched on land, but their efforts were unsuccessful.

What happened to Mr. Sotherden was a mystery that endured for nearly 50 years — until Thursday.

Relying on genetic testing and genealogy research, state troopers in Alaska confirmed that a skull found by the Porcupine River in 1997 was that of Mr. Sotherden. Troopers said in a news release that the suspected cause of death was a bear mauling, but they did not elaborate.

Troopers contacted Mr. Sotherden’s older brother, Stephen Sotherden, in mid-December to ask if he could help confirm a lead in the investigation.

It was the first time since 1977 that investigators had come across new information about what had happened, Stephen Sotherden said in a phone interview on Saturday from his home in Brewerton, N.Y.

“We’ve been working on it for 45 years, and it’s nice that things came to a conclusion,” said Mr. Sotherden, 76. “It was a little more brutal than I was hoping for.”

He said that after his brother was last seen, aerial searches failed to turn up any clues. Their parents hired a mountain guide, who searched for Gary Sotherden by canoeing up the Porcupine River in 1977.

“He did find my brother’s site,” Mr. Sotherden said. “He found his broken glasses. He found identification.”

Mr. Sotherden said his family assumed that his brother had died, around the age of 25, because of the severe weather in the area. The family put a tombstone in the family cemetery plot engraved with his name, Mr. Sotherden said. “Lost in Alaska in the 1970s,” it read.

Mr. Sotherden said that his brother was a “free spirit” who had traveled across the United States and Canada after high school and ended up in Alaska in 1972. Gary Sotherden worked on the Trans-Alaska Pipeline System for a few years before embarking on the hunting trip from which he never returned.

Mr. Sotherden did not know it at the time, but there was a break in the case in April 2022, when investigators extracted DNA from a human skull that had been found 25 years earlier, the Alaskan troopers said.

In July 1997, a hunter found the skull along the Porcupine River — about eight miles from the Canadian border, near where Mr. Sotherden went missing — and turned it over to the police, the troopers said.

But investigators at the time could not find any other remains, and the search for where the skull came from stagnated.

Last year, though, cold case investigators were able to use the extracted DNA and genealogy research to tentatively identify the remains, the troopers said. But to officially confirm that they had found the remains of Gary Frank Sotherden, investigators needed a closer DNA match.

Mr. Sotherden learned that a skull had been discovered near where his brother went missing when he got a call from troopers in December. He was told that DNA testing had matched with their second cousin. A genealogist working with the investigators had then mapped out a family tree between the second cousin and Gary Frank Sotherden and found his older brother, who agreed to DNA testing.

Investigators told Mr. Sotherden that it could take a year to obtain the results, but his wife, Joan, thought that a test he had recently done through the at-home genetic testing service 23andMe could be used for the confirmation. And it was.

“She made it happen,” Mr. Sotherden said. “She really put the pieces together.”

Troopers said the family had been informed about the confirmed DNA match on Dec. 27 and put in contact with the State Medical Examiner’s Office to receive the remains. Mr. Sotherden said his family planned to hold a memorial in the late spring for his younger brother.

“It’s been hard all the way along, but it’s nice to at least know what happened,” he said.