GARDEN CITY, Kan.—Truman Capote's masterwork of murder, "In Cold Blood," cemented two reputations when first published almost five decades ago: his own, as a literary innovator, and detective Alvin Dewey Jr.'s as the most famous Kansas lawman since Wyatt Earp.

But new evidence undermines Mr. Capote's claim that his best seller was an "immaculately factual" recounting of the bloody slaughter of the Clutter family in their Kansas farmhouse. It also calls into question the image of Mr. Dewey as the brilliant, haunted hero.

A long-forgotten cache of Kansas Bureau of Investigation documents from the investigation into the deaths suggests that the events described in two crucial chapters of the 1966 book differ significantly from what actually happened. Separately, a contract reviewed and authenticated by The Wall Street Journal shows that Mr. Capote in 1965 required Columbia Pictures to offer Mr. Dewey's wife a job as a consultant to the film version of his book for a fee far greater than the U.S. median family income that year...

The details are to be found in papers from the Clutter case that a now-deceased KBI agent, Harold Nye, carried home with him years ago. Those documents, reviewed in August by the Journal, are the subject of litigation between the adult son of Mr. Nye, who hopes to publish or sell them, and the KBI, which claims to own the material.

Duane West says the delay is no mystery to him. Mr. West, 81 years old, is the prosecutor who ultimately won convictions and death sentences against the killers, Perry Smith and Richard Hickock.

Mr. West says he remembers well the first time he heard the two suspects' names. It was in the county sheriff's office here, where investigators convened each morning to brainstorm under Mr. Dewey's leadership. On Dec. 5, 1959, when news came that an incarcerated former employee of the Clutter farm had fingered Mr. Hickock, Mr. Dewey delivered a line that his "In Cold Blood" character never spoke.

"Dewey said it wasn't them," Mr. West recalls. "Dewey was convinced it was somebody local who had a grudge against Herb Clutter."

Over the decades, literary sleuths have turned up numerous journalistic sins in "In Cold Blood," ranging from minor inaccuracies to outright fabrication. The latest revelations, though, are particularly damaging because they undermine one of the longest-standing defenses of the book: that the KBI hailed it as true. Mr. Dewey many times called the book accurate.

Mr. Capote's defenders note that the rules of non-fiction-book writing, including the footnoting of source material, hardened only after Mr. Capote helped pioneer the genre. A similar defense can be made for the KBI, which in 1959 had no protocol for how to handle celebrity writers from New York promising not to publish a word until long after the case had been resolved—a vow Mr. Capote kept...

To be sure, the KBI's hesitation in pursuing Messrs. Smith and Hickock was brief, resulting in no delay of justice. Within five months of killing the Clutters, Messrs. Smith and Hickock were caught, convicted and sentenced to death. Both men confessed. They were hanged in 1965.

The agency's assistance to Mr. Capote contributed heavily to the success of a book that became an instant classic, earning its author millions of dollars and acclaim for having created what was hailed as a new literary genre, the nonfiction novel....

The agency has long played down its role in the creation of "In Cold Blood," saying Mr. Capote received no favors or access beyond that granted other journalists. "I never treated Truman any differently than I did any of the other news media," Mr. Dewey said before his death in 1987, in an interview with George Plimpton. "As far as showing him any favoritism or giving him any information, absolutely not. He went out on his own and dug it up."

A substantial body of evidence contradicts Mr. Dewey. In the Capote archives at the New York Public Library, handwritten letters from the author to Mr. Dewey—whom he often called "Foxy"—express gratitude for information and documents received from the agent, along with specific requests for more...