Grann's style is factual but descriptive. One good example occurs in Chapter 1, when Charles Whitehorn's body is discovered:
It was hot and wet and loud on the hillside. Drills shook the earth as they bore through the limestone sediment; derricks swung their large clawing arms back and forth. Other people gathered around the body, which was so badly decomposed that it was impossible to identify. One of the pockets held a letter. Someone pulled it out, straightening the paper, and read it. The letter was addressed to Charles Whitehorn, and that’s how they first knew it was him.
Grann is not making up any central events: Whitehorn really was an early victim of the Osage murders. However, Grann reports the discovery of Whitehorn's body with evocative, detailed description that seems designed to convey more than the simple facts of the case. He describes the hillside as "hot and wet and loud" so that the reader drops into the scene with all their senses. He describes the way oil drills make the earth shake, and the way the derricks (the giant frames supporting the oil drills) move overhead. Their "clawing arms," designed to help break apart the ground and expose the rich minerals inside, evoke the way the murderers too claw for wealth with no care about the destruction they are wreaking.
It is not clear which details Grann is drawing directly from witness accounts and which details he is imagining. For instance, did a witness describe someone straightening the paper they pulled out of Whitehorn's pocket? Most likely at least some of the details come down to Grann's poetic license. The effect, however, is not that Grann becomes an untrustworthy narrator. Rather, the license he takes to fill in details helps the reader get a better sense for what it must have been like to be there on the scene. This is how Grann uses descriptive detail throughout the book. It captures the reader's attention and helps them move into the scene at hand. Even if some of the details are embellished, overall it makes for a book that may in fact be truer to the real events than it would be if Grann were to report only verified details.