According to Seierstad, Bastian Vasquez killed the son of Arfan Bhatti, a leading figure in the Norwegian Islamist group Profetens Ummah, after the boy came to Syria with his mother to engage in Jihad. Vasquez was then executed by ISIS for the crime.
The sensational new claim about the Chilean-Norwegian Islamist, who before his death last year was believed to be one of the most senior European figures in the Islamic State hierarchy, is a side-story to Seierstad's investigation into the two teenage sisters who went from Norway to Syria in 2013.
According to the book, the boy's mother Aisha Shezadi, was first married to Bhatti, but then went voluntarily to Syria where she she became Vasquez's second wife.
Before his death, Vasquez presented some of ISIS's most disturbing propaganda videos. In one, he first shows a group of captives imprisoned in a building, and then shows the building being blown up as he laughs and praises Allah. After the videos were published, Vasquez was charged under Norway's anti-terror laws.
Bhatti as a teenager was a member of Oslo's Young Guns gang, and in 2006 was charged for firing shots at the Oslo Synagogue and the Israeli and US embassies. He resurfaced as an Islamist in 2012.
Åsne Seierstad is most famous internationally for the Bookseller of Kabul, which she published in 2002, but is also known in Norway for her book on the mass killer Anders Breivik, and her reports from Iraq and Chechnya.
The book, released in Norway this week, investigates how two Norwegian-Somali sisters who escaped to Syria in 2013 came to be radicalised, and what has happened to them since they disappeared.
Seierstad believes that they were brainwashed by the man their mother hired to teach them the Koran.
According to Seierstad, the man taught the sisters that it was the duty of Muslims to kill unbelievers, that those who waged Jihad would win a special place in Paradise, and that the West was an enemy of Islam.
“What is central here is that these are teenagers. They are very impressionable,” Seierstad told Norwegian broadcaster NRK. “If this person had entered their life two years before or two years afterwards, it might have not happened."
"There were many others in his class who were not affected,” she continued.
The girls' father travelled to Syria in 2013 to try and bring the two girls back after they ran away from the family home in Bærum, outside Oslo, when they were 16 and 19 years old respectively. Three years later, the girls are still in Syria.
The father, who is not especially religious, told NRK that he agreed with Seierstad's analysis.
“He had an important role. The main role, I think, as a father,” he said. “He quite simply brainwashed our children.”
Asked if he had not brought disaster upon his own family by hiring the man, the father agreed. “Yes, you could say that. It happened, and it's sad.”
According to the book, the girls' mother hired the teacher along with a group of other Somali mothers after he was recommended by someone they knew at Oslo's Tawfiiq Mosque.
The book describes how the girls' brother became increasingly suspicious of the man, and began questioning him on his views.
"You mean that one should kill those who are not Muslims?,” Seierstad reports him asking incredulously. “Those are the words of the Prophet,” the man answered.
According to the book, the brother asked the teacher whether he supported al Qaeda, and the man refused to answer. Then he asked about al Shabaab, the Islamic group which rules parts of Somalia. "I have nothing bad to say about them,” the man replied.
In the book, Seierstad describes how the older sister went from being a top student to wearing the Niqab face veil, against her parents' wishes. Unfortunately, neither her parents nor her teachers had any suspicions of what was going to happen.
“What we know today, did not know in 2013 or 2012, before they left,” Seierstad reminded NRK.