In “The Last Stone,” Bowden focuses on 21 months of questioning by a revolving cast of detectives, telling a stirring, suspenseful, thoughtful story that, miraculously, neither oversimplifies the details nor gets lost in the thicket of a four-decade case file. This is a cat-and-mouse tale, told beautifully. But like all great true crime, “The Last Stone” finds its power not by leaning into cliché but by resisting it — pushing for something more realistic, more evocative of a deeper truth. In this case, Bowden shows how even the most exquisitely pulled-off interrogations are a messy business, in which exhaustive strategizing is followed by game-time gut decisions and endless second-guessing and soul-searching.

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An interrogator’s most important job — like a journalist’s — is to keep the subject talking. “The problem was to convince him that it was in his best interests to reveal what he knew,” Bowden writes, “even though it manifestly was not.” Dave Davis, the detective who spends the most time in the room with Welch, has the softest touch. “Criminals saw the man, not the badge,” Bowden writes; Davis uses empathy as a tactic, even if being empathetic around Welch “meant donning moral blinders.” As another detective with a nurturing style, Katie Leggett, puts it: “We had to endure the ‘friendship’ and go through the crap to get as many of the answers as we could.” Readers will probably recognize a tried-and-true strategy here: good cop/bad cop. When Leggett loses her temper with Welch, still another detective steps in to smooth things over, saying, “I actually think you are still the nice person we thought you were.”

[ Read our review of Bowden’s “Black Hawk Down: A Story of Modern War.” ]

Their interrogation target, is, in Bowden’s telling, an audacious foil, not to mention a colossal narcissist — “natively bright but deeply ignorant and cocky beyond all reason.” Welch seems driven to keep tabs on what the police know. So rather than shut them down entirely, he plays a riskier game, admitting just enough to keep them coming back, but not so much that he could be charged with another crime. His capacity to lie is bottomless, and the lies themselves are rather ingeniously “built around the known facts.” Each time new information contradicts him, Welch alters his story just enough to make sense; each new set of facts resets the cones on the slalom course, and off he glides again, as if on skis. “The ease with which Lloyd made these shifts never ceased to amaze,” Bowden writes. “He acted as if he had never told the story differently.”

Bowden is very good at showing how both sides in this protracted interrogation are lying. Deceit and trickery are tools of the trade for the police, at least in American interrogations. But they are perilous tools. “Once you get into the really interesting stuff,” Bowden writes, “you descend, by necessity, a moral ladder onto slippery ground.” At worst, forcing a narrative onto a suspect can lead to false confessions, as, for example, the wrongly convicted men in the Central Park Five case can attest. And so for a time, we are in a weird area with this book, where the only evidence is what people say, decades later, and the persuasiveness of that evidence depends entirely on whether what those people say can be believed. The more Welch talks, the more the detectives wonder if everything he is saying is simply catering to their own biases. Hours of tall tales, “then five minutes of half-truth,” Bowden writes. “Were the detectives zeroing in on the truth, or was Lloyd just desperately inventing?”

This self-doubt — this perpetual self-scrutiny — is what separates these detectives from, say, the one in Netflix’s “Making a Murderer” who, on video, spoon-feeds a confession to poor Brendan Dassey, a teenage boy so intimidated that he gurgles back what was said to him. (Dassey’s conviction was eventually tossed out, only to be reinstated on appeal to the federal Seventh Circuit.) The best interrogators, Bowden explains, “are connoisseurs of untruth.” Again, Bowden the reporter feels a kinship with them: They, too, must assemble a cogent narrative from a morass of chaos. “What we call history,” Bowden writes, is at best “artful, informed, honest speculation.” The detectives are, in the end, writing a story. Hopefully a true one.