So the targets of radicalized youths are shifting, European intelligence officials said, with terrorist groups either enlisting or inspiring them to attack their homelands. They are employing propaganda tailor-made for youths, including several recent graphic videos showing grammar-school-age children executing prisoners and a newly released computer game, inspired by “Grand Theft Auto,” in which users kill enemies under the Islamic State flag.

Islamic State recruiters carefully monitor children who visit their propaganda sites or enter radical chat rooms, meticulously evaluating who may be suitable for cultivation. Typically, they don’t immediately attempt to challenge children’s relationships with their parents but nudge them toward violence by convincing them that Allah smiles on those who defend the faith. They groom children much the way that pedophiles do — deploying flattery and attention while pretending to be friends, according to people who study the phenomenon.

“They’ve built a structured recruitment process. They’re online, scanning for young adults,” Koehler said. “They have stages of [cultivation]. They won’t even mention violence until later in their contact, until they’ve built up trust with these younger recruits.”

Often, radicalized minors are also children at risk, either suffering from psychological disorders or living in broken or violent homes.