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.

In researching "In Cold Blood," Truman Capote received first-class service from the KBI and Mr. Dewey, its lead detective on the case. Mr. Dewey gave the author access to the diary of 16-year-old Nancy Clutter—her final entry logged only moments before two strangers invaded her home in late 1959 and murdered her, her brother and her parents. Mr. Dewey opened the KBI's case file to Mr. Capote. He pressured press-shy locals to cooperate with the author and granted him extraordinary access to the killers. Mr. Dewey even helped Mr. Capote, a New Yorker with no home in Kansas, obtain a Kansas driver's license.

And Mr. Capote's book painted Mr. Dewey as the investigator who led the KBI's brilliant cracking of the case, and the KBI as a model agency.