Fraihi says these are questions she tried to answer a decade ago because she felt that the radical extremists she heard about were clashing with the kind of Islam she had learned growing up with her parents, who immigrated more than 50 years ago. So she rented an apartment with a divorced woman from Morocco who had been brought over as what was known as “an import bride” to wed someone here. And Fraihi went about searching for answers.

The books preaching jihad that she bought from open book vendors were printed in Amsterdam, but the material had been written in Saudi Arabia. The books advised readers to take revenge on nonbelievers, whether Muslim or non-Muslim. They said that parliament had no authority and that only Allah’s law is sovereign, Fraihi said. And some advised readers to learn how to communicate in symbols and secret code and offered tips on how to do that.

The three youths who later went to Syria she found hanging around street corners. They had quit school and told her they passed much of their time sleeping. Or they would hang around at the metro exits and snatch people’s bags. They called it jihad because they would pick out Westerners. Fraihi told them that sounded like racism. They called it gangster Islam.

They were ripe for picking by international recruiters. “These young people don’t have a job or a future, so they are very easy to indoctrinate if you give them a big story,” she said, “a big collective story, a story of our society, a dream, an aspiration, an idealism.”

Hind Fraihi, a Muslim and journalist who posed as a sociology graduate student, found that she could easily buy extremist literature urging people to take up arms to fight nonbelievers. She met young men being lured from lives of petty crime to violent jihad by local imams. And she interviewed a sheik who sent young men to a military training camp in southern Belgium’s scenic Ardennes and who was recruiting people to fight in Afghanistan and Chechnya.

Three of the young men she interviewed then are now in Syria, as is the sheik, Bassam Ayashi, who was once linked to al-Qaeda. Ayashi, 69, is said to be leading part of the more moderate Islamic Front group fighting the Islamic State and the Syrian government of President Bashar al-Assad; his son and stepson have been killed in battle against Assad forces, and Ayashi was recently injured in a bomb attack on his car.