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,...