But only now is it emerging that Mr. Capote committed an arguably significant act of omission: He neglected to mention in his book or anywhere else that one of his primary sources—the killer Richard Hickock—tried turning into a competitor, by writing his own book-length manuscript about the slaughter of the Clutter family in rural Kansas in 1959.

Mr. Capote’s correspondence from that time shows that he knew about Hickock’s literary effort. And he was worried about it, especially since the murderer finished his manuscript years before the celebrated author completed “In Cold Blood.”

As it turned out, Hickock’s manuscript failed to find a publisher, then seemingly disappeared, rendering it a secret that Mr. Capote took to his grave.

Now, the Hickock manuscript has resurfaced, and it contains an implication that likely would have infuriated not only Mr. Capote but also the Kansas Bureau of Investigation, whose detectives he portrayed so heroically in “In Cold Blood.”

Hickock makes no case for his innocence. Far from it, he describes in bone-chilling detail how he and Perry Smith drove across the plains of Kansas, parked Hickock’s black Chevy sedan in the driveway of the Clutter farm and slipped through an unlocked door into the family home late in the night before Nov. 15, 1959. It describes how they hogtied the terrified Clutters, Herb and Bonnie and their teenagers Nancy and Kenyon, assuring them that they meant no harm right up until the instant that they blasted each in the head with a shotgun.

Hickock gleefully described how, in a house the killers kept dark to avoid the attention of neighbors, he shined a flashlight on each victim’s head while Smith pulled the trigger. Of the instant Smith emptied a shell into the face of 15-year-old Kenyon Clutter, Hickock wrote, “I would of liked to see the embalmer fill that hole.” In the document, the only regret Hickock expressed was that Smith did all the killing.

The twist comes in why they did it.

In Mr. Capote’s telling—which echoed the case prosecutors presented at trial—Hickock and Smith entered the house believing that Mr. Clutter kept $10,000 in a safe, based on a tip Hickock had received from a cellmate during a recent stint in prison. Once they discovered no such safe existed, they could have fled with little chance of getting caught, since the Clutters didn’t know their identities and since limited investigative resources would have been devoted to a robbery that left nobody hurt and that fetched only $40. The murders made no sense. In interviews following the publication of “In Cold Blood,” Mr. Capote theorized that Smith snapped at the sight of an intact family—something he had never had.

In his manuscript, however, Hickock suggested that someone named Roberts paid them to kill one or more of the Clutters. Describing the moment he and Smith parked in the Clutter drive, Hickock wrote, “I was going to kill a person. Maybe more than one. Could I do it? Maybe I’ll back out. But I can’t back out, I’ve taken the money. I’ve spent some of it. Besides, I thought, I know too much.”

Two handwritten pages later, with the Clutters now held hostage, Hickock described a sense of urgency mounting as he and his partner searched the house. “We were running short on time,” Hickock wrote. “It was almost two o’clock and our meeting with Roberts was about an hour away. We didn’t want to miss that. Five thousand bucks is a lot of dough.”

The reasons to discount Hickock’s claim go beyond his lack of credibility as a pathological liar. If he and Smith were paid to kill the Clutters, why didn’t they use that information to try negotiating their way off death row? Why were they dirt poor before and after the crime? Why did Hickock, later in the manuscript, describe the killings with no mention of a Roberts? And how to explain the role of Floyd Wells, the former cellmate of Hickock’s (and former Clutter employee) who testified that his jailhouse talk of a cash-stuffed safe was what got Hickock obsessed with the Clutters?

After reviewing the manuscript at the request of The Wall Street Journal, Michael Stone, a Columbia University psychiatrist specializing in the study of killers, said, “I don’t believe for a minute that they got paid to do it.”

Also improbable is that the Hickock manuscript would have remained undiscovered, and the story surrounding it untold. As a book that created the so-called nonfiction novel, launched the true-crime genre and made Mr. Capote an icon of 20th-century American literature, “In Cold Blood” spawned articles, books and films galore. Two years ago, the Weinstein Co. optioned television rights to “In Cold Blood,” and last fall, SundanceTV announced plans for a four-part documentary on the Clutter murders.

Yet the story of Hickock’s literary project has escaped attention of even Capote scholars, even though Hickock’s partner in that project was the late Mack Nations, a prominent Kansas journalist. “I didn’t know about any Hickock manuscript, and I’d never heard of Mack Nations,” said Ralph Voss, who spent years researching his acclaimed 2011 book, “Truman Capote and the Legacy of In Cold Blood.”

Officials of the KBI and Kansas attorney general’s office declined to respond to questions about the Hickock manuscript, or any suggestion that a person named Roberts ordered the killings.

In 1962, Mr. Nations sent to Kansas authorities a copy of the manuscript he had received from Hickock. That fell into the possession of Robert Hoffman, a lawyer in the Kansas attorney general’s office who successfully batted down Hickock’s and Smith’s appeals for clemency. When he died, the copy passed to his son, Kurt Hoffman. After the Journal learned of it, the younger Mr. Hoffman agreed to let the Journal review the manuscript.

A look at the history of the Hickock/Nations project suggests that its obscurity isn’t necessarily an accident. Documents on file at the Kansas Historical Society in Topeka and the New York Public Library, along with letters of Mr. Capote and interviews with people who knew Mr. Nations, suggest that Kansas prison and law-enforcement officials sought to thwart the Hickock/Nations book while enabling Mr. Capote’s.

“Capote was telling the story that Kansas authorities wanted told, and Mack Nations was telling a story that they wanted to silence,” said Houston probation officer Michael Nations, son of the late Mack Nations.

In early 1961, a year after Hickock and Smith were convicted of killing the Clutters, Mack Nations visited the Kansas State Penitentiary in Lansing to write stories about death row for his employer, the Wichita Eagle. While he was interviewing Hickock the idea of a book arose, and ultimately they agreed to a joint venture. Hickock would write his story and mail it in installments to Mr. Nations, who would fashion it into a book-length manuscript, according to the younger Mr. Nations and to prison correspondence on file at the Kansas Historical Society.

Over the following months, Hickock sat on death row writing about 200 pages by hand, putting his initials in the top corner of some pages. Whether he saw himself as competing with Mr. Capote isn’t known. At that point, Hickock hadn’t seen Mr. Capote in more than a year, not since the author had traveled to western Kansas for the Clutter-murders trial. In the film “Capote,” the author is portrayed as a constant visitor to death row. In fact, an execution date and almost two years passed without Mr. Capote paying any visits to the eastern Kansas prison where Smith and Hickock awaited the gallows, documents at the Kansas Historical Society show.

It was only after KBI Detective Alvin Dewey, chief investigator in the Clutter case, wrote to Mr. Capote in May 1961 informing him of the Hickock-Nations project that Mr. Capote made plans to visit death row, according to the letter, now in the Capote collection at the New York Public Library. By then, however, he faced an obstacle: A few months after Hickock began sending his story to Mr. Nations, Kansas prison officials imposed a new ban on visits and correspondence between the media and death-row inmates. The reason for the ban isn’t clear.

Mr. Capote hired a powerful Kansas law firm to seek an exception for himself to the ban, according to documents at the Kansas Historical Society. That effort was successful, and in January 1962 Mr. Capote paid his first visit to Smith and Hickock at the penitentiary.

That same month, state prison officials requested from Mr. Nations a full copy of the raw manuscript he had received in installments from Hickock. Mr. Nations complied on Feb. 1, 1962, although in a cover letter to Mr. Hoffman in the Attorney General’s office he expressed dismay that the ban prohibiting him from visiting or corresponding with Hickock had been lifted for Mr. Capote. “I can’t help but feel the attorney general’s office played a little politics in opening the penitentiary to Truman Capote while all other reporters, including myself, are prohibited from making such visits,” he said in the letter, acquired by Gary McAvoy, a Capote scholar and the author of a coming book about the Clutter case.

Meanwhile, Mr. Capote tried getting his own copy of the Hickock manuscript, according to Shirley Wise, then a general assignment reporter at the Wichita Eagle. “Capote called the Eagle asking for Mack Nations, and I answered the phone,” said Ms. Wise, who was also Mr. Nations’ girlfriend back then. “I transferred the call to Mack, and I heard him say, ‘Hell, no!’ Mack told me Capote wanted to buy the manuscript from him.”

Alan Schwartz, a California attorney who represents the Capote estate, didn’t return emails and calls seeking comment.

By that time, early 1962, Mr. Nations had fashioned Hickock’s writing into a book and sent it to Random House, apparently unaware the publishing house already had signed a contract with Mr. Capote to write about the Clutter murders.

Soon after, when a package came back from the publisher, Mr. Nations handed it to Ms. Wise and asked her to open it, she said. “When I asked him what it was, he said, ‘It’s my retirement,’ ” said Ms. Wise, now an 81-year-old insurance agent in Wichita. But it was a rejection. “As I recall, the letter said Random House already had Capote under contract,” she said.

A month after Mr. Nations gave state officials the copy of Hickock’s letters, federal prosecutors in Topeka brought income-tax evasion charges against the journalist. The case was related to an alleged payoff to Gov. Fred Hall that Mr. Nations was accused of handling in 1955, when he was in an earlier job as the politician’s executive secretary. The $10,000 bribe was allegedly paid by Kansas lawyers to arrange parole for someone who had performed an illegal abortion that resulted in death. Mr. Nations was charged with tax evasion on his alleged $5,000 cut.

When Mr. Capote learned, via the KBI, of the indictment against Mr. Nations, he expressed delight. “Remember Mack Nations, the newspaper b*****d who has caused me so much trouble?” he wrote in a March 1962 letter to Random House founder Bennett Cerf. “Well, he has been arrested for income tax evasion!” The letter was reprinted in “Too Brief a Treat: The Letters of Truman Capote,” a book edited by Capote biographer Gerald Clarke and published in 2004.

Mr. Nations was acquitted after a trial. But his career at the Wichita Eagle was over, and legal expenses had ruined him financially, according to his son. Mr. Nations moved to Colorado, took charge of a small newspaper and died in a single-car accident in 1968, two years after “In Cold Blood” became an international sensation.

After its rejection, Mr. Nations’ manuscript remained for years with Ms. Wise. Eventually, Ms. Wise said she delivered the manuscript to Mr. Nations’ mother. Called “High Road to Hell,” the manuscript found its way back to him in Colorado, where his son recalled seeing it on his father’s desk shortly before his death. “After he died, nobody ever saw it again,” said Michael Nations.

The journalist’s son believes it is no coincidence that the federal charges against his father came a few weeks after state officials received a copy of the Hickock manuscript. “They trumped it up to put a stop to that book,” said the younger Mr. Nations, who in recent months has posted several long YouTube videos in which he theorizes that the Clutter killings might have been a paid hit and his father the victim of a coverup.

The only piece of Hickock’s writing that Mack Nations ever published was a short story in a long-defunct magazine called Male in late 1961, in which Hickock came off as unrepentant and repugnant. The Male magazine story acknowledged that Smith and Hickock killed the Clutters but made no mention of anyone paying them to do the killing. Smith and Hickock were hanged in 1965.

Ms. Wise said Mr. Nations once remarked to her that “there was something fishy about the Clutter case. But he wouldn’t say what.” She described the late journalist as a gregarious raconteur who also had a penchant for secrecy.

In the lengthy section of “In Cold Blood” devoted to the Clutter killers’ five years on death row, Mr. Capote portrayed Hickock as a vain hood who spent his time reading erotic literature and legal manuals on how to seek clemency. He never mentioned that Hickock tried publishing his own book about the case.