"Two Sisters," a riveting and masterfully constructed book of reportage by Norwegian journalist ├ůsne Seierstad, attempts to answer a question that much of the world is asking: How does a normal teenager become so radicalized that she leaves the safety and comforts of home for a chaotic future of danger and depredation?

For Ayan, 19, and Leila, 16, two Somali immigrants in a loving, stable family in Norway, that uncertain future was with the Islamic State in Syria. The girls' mother and father, Sara and Sadiq, never saw the radicalization evolve, and were devastated when the girls left suddenly in the fall of 2013. Yet all the signs of the transformation were there well in advance.

In part because of a Muslim organization in Norway called Islam Net, the girls experienced "a profound religious awakening." They became more conservative in their beliefs, shunning Western-style clothes and music. They sequestered in their bedroom, forming alliances on social media. They covered their faces with niqabs and attended lectures that promoted a literal interpretation of Islam with an emphasis on resistance to Bashar Hafez al-Assad's brutal counterinsurgency in Syria's civil war.

However, Islam Net also attracted a sinister element. "The differences between those rougher-looking types and the leadership of Islam Net could appear slight from a purely religious point of view," Seierstad writes. "What set them apart was a belief in violence." Those jihadists became very attractive to these two impressionistic teenagers.

"Two Sisters" also tracks Sadiq's frantic journey to rescue his daughters. He travels repeatedly to Syria, depletes his family's finances and is briefly captured by the Islamic State while witnessing the group's brutal murders. Seierstad's dramatic narration of Sadiq's search infuses "Two Sisters" with a desperation that will resonate with any parent.

Yet the daughters, who communicated sporadically to their family by phone, e-mail and Facebook, did not want to be rescued. Ayan and Leila had no intention of ever returning to Norway, where 36,000 Somalis comprised the largest non-Western minority in a most accommodating country. They eventually married Islamic State fighters and settled in Raqqa, Syria. At the end of the book their fates are unknown.

Seierstad's exhaustive reporting mines original sources, such as texts and other forms of messaging, but she does not include any interviews with Ayan or Leila, neither of whom would grant permission to the author to write about them. The author defends her decision to go forward without consent.

"The entire world is trying to understand the reasons for radicalization among Muslim youth," she writes.

"Two Sisters" offers readers that understanding without judgment, in a manner only great journalism can accomplish.

Stephen J. Lyons is the author of four books, most recently "Going Driftless: Life Lessons From the Heartland for Unraveling Times."

Two Sisters: A Father, His Daughters, and Their Journey Into the Syrian Jihad
By: Asne Seierstad.
Publisher: Farrar, Straus & Giroux, 418 pages, $28.
Event: 7 p.m. April 16, in conversation with Fred de Sam Lazaro. American Swedish Institute, 2600 Park Av. S., Mpls., co-hosted by Norway House and Rain Taxi. Tickets $5, raintaxi.com/asne-seierstad/ or at the door.