Capote had to make the good choices about the structure of the book because when In Cold Blood was first published in January 1966, Hickok and Smith had been dead for less than a year. The murder and trial had hit the headlines, and many readers probably knew the details of the novel before they began reading it. Capote had to make it interesting even to people who knew the outcome--the book had to be good literature as well as be informative and accurate. Capote chose a starting and ending point, and in between he chose the order of the passages.
The book is organized into a basic structure of narrative, in three parts: the facts that begin the story, the rising action, and the resolution.
I will explain those three parts of the structure:
The first part, with I/The last to see them alive (from the first page to page 85): which is the beginning, the story itself, with the settings and the writer introduces the characters: we learn about 2 sets of characters: The Clutter Family and the 2 parolees. So we see the Clutter live their last day together on the farm and in the community, we get to know their solid Midwest values. And we get information about the two guys who have decided to rob and murder the Clutter, we begin to learn about their background, and this first part ends with the news of the murders.
and we have the second part, which includes II/Persons unknown and III/ Answer from page 89 to 286 : those two middle sections show the rising action. We get to know about the clean-up of the Clutter house, investigators start to look for clues to solve the murders. On a parallel track, the killers flee Kansas to pursue their dream of life in Mexico, and then they are back to the US (that was for “Persons unknown”) and then in “Answer”, the parallel tracks begin to converge and somebody, Wells, an ex-inmate reveals that Hickok vowed to commit the murders. The killers turn violent, finally