This admission is tucked away at the very end, in an afterword. It explains the prickling feeling you might have while reading the book that information is being withheld, that Seierstad knows more than she’s telling. Sadiq’s influence over the text also explains why he is such an outsize figure in the narrative, and perhaps why his account is not subject to more scrutiny, given what an unreliable narrator he is — particularly, Seierstad shows us, where journalists are concerned.

At one point, Sadiq interested documentary filmmakers in his story, telling them his daughters had begged him to rescue them (untrue). When the man he hired to kidnap them back refused the job, Sadiq concocted a fantastic story — that the hired man was caught, crucified and beheaded. It was easier than admitting to the filmmakers that he had lost control of his daughters, that “he had failed as a father and as a man.” This sort of thing — these lies to save face — occur with troubling frequency, and it’s strange that Seierstad refrains from commenting on them, especially since her own book is built on Sadiq’s testimony.

Seierstad is at her best when she pans out to consider the variety of reasons Western women join ISIS (by 2013, there were 3,000 Westerners in Syria, several hundred of them women), drawn by a hunger for sisterhood, adventure and membership in a society they felt was colorblind — where shared allegiances were more important than race. She evokes the life they find in ISIS-controlled territory in Syria — where boys are taken from their parents at 10 and placed in camps to be trained in combat, their final test being an execution. She quotes message boards where women wrote about their new lives: the houses given to fighters for free; groceries and stipends delivered monthly; free medical care. The sisters boasted to their brother about their large house and garden, their pet rabbits. As Sadiq survived imprisonment in a fetid latrine, the daughters he was risking everything to save were making pancakes in Raqqa, with sugar topping.

But in general, the sisters and their transformations remain hazy, especially the strong-willed Ayan. At one point, her school essays sound like outtakes from “SCUM Manifesto,” by the radical feminist Valerie Solanas. In a piece entitled “Women’s Liberation,” Ayan wrote: “What’s worse, we have to give birth to little rat males who we look after and hold dear, right up until they turn from boys into men who in turn go on to oppress yet another woman.” And, yet, a few years later she defended the rape of Yazidi and Kurdish women by ISIS fighters: “They’re not women,” she told a friend. “They’re spoils of war.”

In her book “The Brothers,” about the Boston bombers Tamerlan and Dzhokhar Tsarnaev, Masha Gessen points out that religiosity has little to do with radicalization. Extremism more often follows crises in identity and in community, when other narratives of making sense of the self have fallen away. This is what becomes obvious every time the sisters in Seierstad’s book flicker into focus, when their voices can be heard, unmediated. This story of theirs has yet to be told — despite the resonant clues they left along the way — and even as it promises to be repeated by others.

Ayan once wrote a fairy tale in school about “two young girls who wanted to go out into the world and find themselves.” “At home they had always got everything they wanted, attended the best school, worn the newest clothes, but had never been given the opportunity to decide anything for themselves, something young women strongly desire,” she wrote. “Late one summer evening, when their parents were not at home, they wrote a text message: ‘Mom, Dad, we’re going out for a while, don’t wait up.’”