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A soft-spoken records and filings supervisor here may have answered one of the biggest unsolved mysteries of the Wild West: Who killed the legendary lawman best known for fatally shooting Billy the Kid?

Angelica Valenzuela, a clerk for Dona Ana County in southern New Mexico, said she was sifting through boxes of obscure records when she came across a faded document identified as the 1908 coroner's report for the death of Pat Garrett.

Garrett is credited with shooting the legendary outlaw Billy the Kid, whose real name was Henry McCarty, in what some called an ambush at Fort Sumner, N.M. on July 14, 1881.

Valenzuela found the long-sought document when she was carrying out a massive effort to preserve fragile documents spanning from the waning days of the Wild West in the mid-1800s up to the 1960s. That region of the southwest is where lawmen frequently chased outlaws and clashed with renegade Native American tribes such as the Apaches.

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“I was with a co-worker going through boxes when I came across coroner’s jury reports,” said Valenzuela. “I said let’s read this one together. And we looked at each other and our eyes got wide and our mouths dropped.”

She said they burst out laughing but then realized the importance of the document.

“We were holding history in our hands,” she said. “It was really exciting.”

They were holding Garrett’s official cause of death and by whom, a subject of great controversy for more than a century.

The handwritten document signed by seven jurors read, in part: “We the undersigned Justices of the Peace and Coroners Jury have attended the investigation of the body of Pat Garrett who was reported dead within the limits of Precinct No. 20, County of Doña Ana, territory of New Mexico on about five miles northeast of the town of Las Cruces and find that the deceased came to his death by gunshot wounds inflicted by one Wayne Brazel.”

Just two months prior to the startling discovery, a historian had requested the documents and Valenzuela’s office could not find them. There was speculation they were destroyed to mask a secret and keep the public in the dark as to who killed Garrett.

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That researcher was Arizona State University Professor Emeritus Dr. Robert J. Stahl, who estimates the value of the document in the tens – perhaps hundreds – of thousands of dollars if it were to be sold on the open market,

University of New Mexico history professor Paul Hutton, an expert on the Wild West, said the controversy continues despite the discovery of the document.

This undated file photo is thought to be an image of famed gunslinger Billy the Kid, William Bonney, near the age of 18

This undated file photo is thought to be an image of famed gunslinger Billy the Kid, William Bonney, near the age of 18 (AP)

“Almost all historians agree that Brazel probably did not kill Garrett. Or, if he did, he had help. But why was Garrett killed?” Hutton asked. “Was he actually getting close to solving the Fountain murder case (the unsolved killing of politician Albert Jennings Fountain) or was he threatening a local smuggling operation (one historian says that he was about to expose local ranchers who were smuggling illegal Chinese workers). Or was it a land dispute? Still a mystery. One thing is not a mystery and that is that the fix was in to acquit the addled Brazel of Garrett's killing.”

Garrett’s law enforcement career ended after a series of failed elections. He then settled into a life as a rancher outside Las Cruces, though he drank heavily and was under severe financial straits.

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In an effort to defray his debt to a neighbor, Garrett agreed to rent one of the man’s employees, Brazel, a parcel of land.

As happened frequently to the legendary lawman, things did not go smoothly.

On Feb. 29, 1908, Garrett, Brazel, and a third man met up on a trail near the ranch. An argument ensued and Brazel allegedly shot Garrett in the back of the head and the stomach. Brazel would admit to the murder, saying it was self-defense. But the sheriff didn’t buy it. He was put on trial for murder but Brazel was found not-guilty after the jury deliberated for 15 minutes.

Hutton said even by New Mexico’s dubious standards of justice, shooting a man in the  back while he was urinating on the side of the road, which Garrett was actually doing at the time of his murder, and getting off on self-defense is pretty amazing.

For more than 100 years, there was speculation and conspiracy theories as to who actually killed Garrett. In the waning days of his law career, Garrett investigated a double homicide that secretly concerned many prominent people in the area.

One popular theory had Deacon Jim Miller, one of the Old West’s best known assassins, as the actual hitman.

That is, until Valenzuela’s inadvertent discovery.

Garrett and Billy the Kid actually were friends at one point, but that soured when the southern lawman was tasked with tracking down the outlaw in connection with the murder of a crooked sheriff and his deputy in 1878.

After nearly three years on the run, Garrett finally captured Billy the Kid and put him in the Lincoln County Jail. The Kid was sentenced to hang for the murders but two weeks before the execution date, he made a daring escape from the jail – killing two sheriff’s deputies and further fueling his reputation as a dangerous bandit.

On the night of July 14, 1881, Garrett finally tracked the Kid down to the Maxwell Ranch near Ft. Sumner and shot his 21-year-old suspect in a dark room. The shooting would be considered an ambush by some but Garrett was found legally within his rights to kill the outlaw.

Valenzuela said the county will be holding an event on June 16 to display this and other historic documents.