NEW YORK—At a flea market six years ago, a North Carolina lawyer named Frank Abrams unknowingly bought a rare photograph that experts say shows Billy the Kid relaxing with the man who would eventually kill him.

Billy the Kid, one of the best-known outlaws in American history, is thought to be there in the back, second from the left. The man all the way to the right is Pat Garrett, who would soon become the sheriff of Lincoln County, N.M., and — the story goes — shoot the outlaw dead in a darkened room.

The men appear on a tintype that is about the size of a man’s palm. (Tintypes are photographic images produced on thin sheets of metal. They became popular during the late 19th century and, as in this case, often show a version of reality that is reversed, left-to-right.)

In 2011, Abrams saw nothing more than a group of five men who looked like cowboys. He considered it a strange find, since most tintypes that ended up in North Carolina hearkened back to the American South, not the Wild West. So he bought it for $10, he said in a phone interview, and put it up in his home. It hung in a room where he hosted Airbnb guests. Abrams used to jokingly tell them that it was a picture of Jesse James.

A similar find — a tintype that experts said showed Billy the Kid playing croquet with friends — was valued at around $5 million in 2015. The discovery motivated Abrams to take a closer look at his own picture.

He turned to Google and eventually zeroed in on the man on the right with the severe features and the dark hat. “Oh my gosh,” he recalled saying. “That is Pat Garrett in my picture.”

Then, Abrams began to wonder about the man in the back with the prominent Adam’s apple. He eventually showed the tintype to Robert Stahl, a retired professor at Arizona State University and an expert on Billy the Kid.

Stahl encouraged Abrams to show the image to experts.

William Dunniway, a tintype expert, said the photograph was almost certainly taken between 1875 and 1880. “Everything matches: the plate, the clothing, the firearm,” he said in a phone interview. Dunniway worked with a forensics expert, Kent Gibson, to conclude that Billy the Kid and Garrett were indeed pictured.

Stahl said that Garrett and Billy the Kid, who also went by William H. Bonney and Henry McCarty, were friends who once gambled together. But when Garrett was about to become the sheriff of Lincoln County, he urged the outlaw to hit the road.

“Garrett was saying, ‘If you leave New Mexico, I’m not going to pursue you. But if you stay in the territory, then no matter where you are, I have to come after you,’” Stahl said. That would have been around the time the photo was taken.

Once he became sheriff, Garrett’s men did indeed capture Billy the Kid. But the outlaw escaped, killing two deputies on the way out of jail. So Garrett tracked him down again. The story goes that in 1881, Garrett was in Fort Sumner visiting a friend of the outlaw’s when the Kid arrived unexpectedly; the men couldn’t really see each other, but Garrett recognized Billy’s voice and quickly shot him dead.

Reporting on his death in 1881, The New York Times described Billy the Kid as slim, good-looking and mild-mannered.

“His soft blue eye was so attractive that those who saw him for the first time looked upon him as a victim of circumstances,” it reported. “In spite of his innocent appearance, however, Billy the Kid was really one of the most dangerous characters which this country has produced.”

Billy the Kid became famous for his participation in the Lincoln County War in 1878, a bloody feud between rival factions in New Mexico Territory. But the story of his short life — he was said to be around 21 when he died — has always been a subject of fervent debate and a few conspiracy theories. People have disagreed about his motivations, about the total number of men killed by him and his crew, and even about whether he really died that night in Fort Sumner.

One of the people who tried to set the record straight was Garrett himself, who worked with the journalist Marshall Ashmun Upson to write a book: “The Authentic Life of Billy the Kid.”

Abrams said he bought the tintype from people who told him it came from Clinton, N.Y. He believes that it had been a possession of Upson and had found its way to his relatives in the Northeast.

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And then it made its way to Abrams, who is, of all things, a criminal defence lawyer. “It was like taking on the biggest case you could ever imagine,” he said of investigating his artifact, which is now in a safe-deposit box.

He has not had the tintype formally valuated, and he said that for now, he is not concerned about its monetary value. “Other people, they want to speculate from here to kingdom come,” he said. “I don’t know what it’s worth. I love history. It’s a privilege to have something like this.”