An Iraqi soldier patrols a suburb close to Ramadi, the capital of Iraq's Anbar province. Iraqi forces declared victory there in December after wresting control back from the Islamic State. (Ahmad Al-Rubaye/AFP/Getty Images)

Mary Louise Kelly is NPR’s national security correspondent. Her latest novel is “The Bullet.”

Confession: I burst out laughing when I read the jacket copy of Peter Bergen’s new book. Not because of the title — “United States of Jihad: Investigating America’s Homegrown Terrorists” isn’t what you’d call a rib-tickler. And not because of the intriguing and timely premise. Bergen seeks to explore why some Americans are drawn to jihad and how the threat of terrorism on U.S. soil has changed our country and its institutions.

No, the bit that got me chuckling was the pledge that Bergen’s narrative would be “paced like a detective story.” When did it become de rigueur for books on serious security topics to promise such a thing? A sample check of my personal library reveals recent nonfiction tomes on the CIA (“As chilling and dramatic as a spy novel — except it’s true”), on a Russian double-agent (“Fast-paced and exciting as the best spy novel”) and on nuclear terrorism (“It reads like a thriller, but it is true!”). You learn to take such declarations of sizzle with a large grain of salt.

What a pleasure, then, to discover that Bergen actually delivers. “United States of Jihad” sketches succinct cameos of the Americans who have embraced militant Islam. Nearly 300 Americans have been indicted or convicted on terrorism charges since 9/11; Bergen focuses on the big names, from Fort Hood shooter Nidal Hasan, to Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev, to Anwar al-Awlaki, the cleric who became the first American killed by a CIA drone.

The stories are grim, and of course — we know how they end. Still, Bergen pulls you in with snappy, conversational writing. (Sample chapter opening: “In the tense months following the 9/11 attacks, Anwar al-Awlaki was feeling particularly horny.”) Early chapters set about exploding some of the easy assumptions about jihadists in the United States. It’s natural to suspect that the decision to turn to terrorism must be rooted in some traumatic life experience or that these people must be pathologically disturbed, or career criminals, or just plain dumb. Not the case. “They are, on average, as well educated and emotionally stable as the typical citizen,” Bergen argues.

They are, in other words, ordinary Americans.

So what motivates them? If you’re looking for a portrait of the typical terrorist, one lesson here is that there may be no such thing. Take the newest generation of would-be terrorists. Bergen notes that as of last summer, more than 50 American citizens or residents had been charged with joining or attempting to join either the Islamic State or Jabhat al-Nusra, al-Qaeda’s affiliate in Syria. They include a 34-year-old white man and registered sex offender from Wisconsin, a 47-year-old African American man who once served in the Air Force, and a 29-year-old white woman from Chattanooga, Tenn., who was raised an evangelical Christian.

A rare common thread is that these recruits skew younger and more female than you might expect. One in five are teenagers, including six girls — a trend Bergen attributes to the Islamic State’s reliance on social media as a recruitment tool. Here is a generation glued to their phones, immersed in a virtual community of like-minded militants. Bergen recounts the efforts of one frustrated cleric, Imam Mohamed Magid, who presides over one of the largest mosques in America, the All Dulles Area Muslim Society in Northern Virginia. Magid counsels Islamic State-enamored teenagers, challenging them to question whether its members are true Muslims. But as soon as he finishes, militant recruiters swarm in with tweets and texts. “Magid would talk to this youngster for two hours,” Bergen writes, “and ISIS would then ‘undo him after two hours, three hours, four hours.’ ”

Bergen makes the case that the real threat from the Islamic State will remain “lone wolves” — Americans inspired by the group, rather than directly financed or trained by it. And he frets about the convergence of the lone-wolf phenomenon with the Islamic State’s social-media savvy. As Bergen memorably puts it, “The lone wolf is now part of a virtual pack.” It’s not an original insight, but it is an important one.

What, then, is to be done? How to slow the pipeline of new recruits to the Islamic State? And how to prevent a large-scale attack like the one in Paris from happening here? Unfortunately, the prescriptive sections are among the weakest of the book. There is a long and not particularly compelling discussion of the FBI’s behavioral analysis unit, which seeks to identify when someone may be on a “pathway to violence.” There is a call for collective vigilance, for everyone from strangers to family members to sound the alarm on plots in progress. “Early intervention seems, now more than ever, an avenue worth exploring,” Bergen writes.

As for the high-profile tools the U.S. government has deployed to counter terrorism since 9/11 — National Security Agency surveillance, drone strikes, FBI sting operations — Bergen does an able job sifting through the legal and ethical questions they raise. Is it legal, for example, to kill a U.S. citizen without due process, as happened when Awlaki was targeted by a CIA drone? Obama administration lawyers decided yes, but you sense that Bergen is still struggling with the question. (And as he points out, on a purely practical level, “killing Awlaki . . . [did] little to kill his ideas.”)

Bergen saves his big takeaway for the very last page, and it is this: Jihadist terrorism does not pose an existential threat to the United States. Only three crises have, in his view. They were the Civil War, which threatened to break the nation in two; World War II, which could have meant the end of Western liberal democracy; and the Cold War, which could have ended in mutually assured destruction. The attacks on 9/11 — however terrifying, however tragic — amount to a lesser catastrophe. What we are left with, these 15 years later, is a “persistent low-level threat that will likely take many, many years before it withers and dies.”

United States of Jihad
Investigating America’s Homegrown Terrorists

By Peter Bergen

Crown. 387 pp. $28