What prompts someone to convert to Islam and to sign up for global “holy war” in the name of Osama bin Laden’s al Qaeda? Security agencies are asking that question with increasing urgency as they confront a growing catalog of actual or attempted attacks in which Muslim converts are suspected of playing prominent roles. Christian Ganczarski, a German suspected of involvement in a 2002 bombing in Tunisia, converted at 20 before embarking on a jihadist career in which, investigators believe, he became a close associate of bin Laden’s. Other high-profile militant converts include Jamaican-born Germaine Lindsay, one of four suicide bombers who killed 52 people in London in July, and Briton Andrew Rowe, jailed for 15 years last month for possessing terrorist materials.

Frenchman Lionel Dumont, a suspected Rowe associate and another convert, will go on trial in December accused of a series of attacks in the 1990s, including an attempt to bomb a Group of Seven summit in Lille.

“It’s striking, the number of converts engaged in terrorist activities,” said Michael Taarnby, a researcher at the Danish Institute for International Studies who has studied the recruitment and radicalization of Islamist militants. Jean-Louis Bruguiere, France’s top anti-terrorism judge, told the newspaper Le Figaro in an interview: “The converts are undeniably the toughest. Nowadays the conversions happen more quickly and the commitment is more radical.”

The phenomenon is not confined to Europe. John Walker Lindh, dubbed “the American Taliban,” was convicted and jailed in 2002 for fighting alongside the Afghan militia, and US citizen Jose Padilla has been held for more than three years as a suspected enemy combatant in connection with an alleged “dirty bomb” plot. In Australia, British-born Muslim convert Jack Roche was jailed for nine years in 2004 for conspiring to bomb the Israeli embassy in Canberra.

<B>“American taliban”</B>

In interviews with Reuters, European experts said the vast majority of those who converted to Islam did so for legitimate personal reasons. Some convert in order to marry Muslims. Many converts were drawn, the experts said, by the appeal of a universal faith that transcended national and ethnic barriers, offered a sense of belonging and brotherhood and provided a new identity, including the choice of a Muslim name.

However, a small fraction were extremists who saw in radical Islam a vehicle to challenge and overthrow the existing world order, said Olivier Roy, research director of the French National Center for Scientific Research. “If you are a youngster in the French suburbs, your mates are second-generation Muslim immigrants and you want to wage war against society, the system, where do you go?” said Roy. “Thirty years ago, you joined the Maoists, the Trotskyists, the far left, the Baader group, Action Directe. Today, where do you go? Bin Laden.”

A German intelligence official cited cases where radical foreigners had acquired residents’ status by marrying local women, complicating authorities’ attempts to kick them out. “It gives them more security in their legal status. If they’re married to a German woman, it’s very hard to expel them,” he said.

Some of the best-known extremist converts whose cases have come to trial were drifters on the margins of society. David Courtailler, a Frenchman convicted last year of abetting terrorists, was drawn into radical circles when he converted to Islam at a British mosque and was approached by a stranger there who gave him money and an air ticket to Pakistan.

<B>Jon BOYLE

Mark TREVELYAN</B>