The breeding ground for much of this was 4chan, the “Deep Web” destination “still mostly unknown to the mainstream but beloved by millions of regular users.” The realm of 4chan called /b/ is where some of this book’s most destructive characters spent their early Internet years, soaking up so much pornography, violence and in-joke humor that they became bored enough to move on. Ms. Olson, whose evenhanded appraisals steer far clear of sensationalism, describes 4chan as “a teeming pit of depraved images and nasty jokes, yet at the same time a source of extraordinary, unhindered creativity.” It thrived on sex and gore. But it popularized the idea of matching funny captions with cute cat photos too.

“We Are Anonymous” also captures the broad spectrum of reasons that Anonymous and LulzSec attracted followers. Some, like Topiary — who turned out to be Jake Davis, an outwardly polite 19-year-old from a sheep-farming community on the remote Shetland Island called Yell, who was arrested in 2011 — were in it for random pranks and taunting laughs. This book does not shy away from the raw language its principals used, as when Topiary told one victim: “Die in a fire. You’re done.” Other participants had political motivations. The New Yorker calling himself Sabu began as a self-styled revolutionary and was instrumental in getting Anonymous to invade the Web sites of top government officials in Tunisia.

A pivotal part of this book concerns the arrest of Sabu, the unveiling of his real identity as Hector Monsegur, and the F.B.I.’s subsequent use of him as an informant. Sabu’s dealings with Julian Assange of WikiLeaks are also described. Ms. Olson notes how Sabu “suddenly seemed very keen to talk to the WikiLeaks founder once his F.B.I. handlers were watching.”

Ms. Olson regards it as inevitable that neither Anonymous nor LulzSec could reconcile the divergent goals of its participants. Bullying jokesters and politically oriented hacktivists may share sophisticated knowledge of how to manipulate the Web and social media, but each faction became an embarrassment to the other. Topiary told Ms. Olson about his own long-distance contact with Mr. Assange, whom he describes as both intrigued by the saboteurs’ potential and critical of their silly side. (After sifting through 75,000 e-mails from a digital security firm, Topiary bashfully admits, one of the things that most interested him was an e-mail from the chief executive’s wife saying, “I love when you wear your fuzzy socks with your jammies.” )

The most startling conversation in “We Are Anonymous” was arranged by the author: an in-the-flesh meeting between Topiary and a person she calls William, since he remains unidentified.

William personifies the dehumanizing effects of cybercrime, and he knows it. One of his specialties is extorting pornographic pictures and then putting them to damaging use. “We split up several boyfriends and girlfriends and appalled many people’s mothers,” he recalls, about the Facebook tricks the book describes in detail. “I’d be lying if I said there was any great reason,” he adds. “I don’t feel guilty, it makes me laugh, and it wastes a night.”

Together they confirm the worst suspicions about the power of sophisticated but untethered Internet manipulation. “You could inspire some 15-year-old, or someone with a 15-year-old’s mind-set, to hate whoever you want them to hate,” William says.