One lesson, they said, is that former extremists have a central role to play in the argument against radical temptations. They have a credibility that governments lack.

“We need to replace fantasy with reality,” said Amy Thornton of the Department of Crime and Security Science at University College London. “Formers play a very important role. Only they can credibly say: Syria is not a video game, you may end up cleaning toilets, babysitting on the front line; it’s not what you’re being promised.”

Another lesson, experts say, is that debunking extremist propaganda alone is not enough. Outreach efforts are most effective, they said, when they offer a counternarrative and tangible alternatives to violence.

One pioneering program in Denmark treats onetime fighters not as potential terrorists but as wayward youths. Closely watched by the authorities around Europe, the program involves counseling, help with readmission to school and meetings with parents. Although now being applied to Islamic radicals returning from the Middle East, it was first developed in 2007 for far-right extremists.

There are limits to the willingness of governments to rely on such a program. But experts in radicalization said that understanding the process by which people fell for the medieval brutality of a religious ideology is vital to combating it.

“We won’t make any progress at all if we continue to obsess over the question ‘why’ someone becomes an extremist,” Mr. Horgan said. “A better starting point is asking ‘how.’ ”

For Mr. Orell, now 34, it started on a summer evening in Stockholm in 1995.

Then an anxious 14-year-old with divorced parents and difficulties in school, he suffered regular intimidation by a gang of boys from immigrant backgrounds. The only place he felt safe was with a youth club, where he discovered punk rock with lyrics that spoke of Viking conquest.