What could drive two German men called Fritz and Daniel to convert to Islam and then, according to prosecutors, embark on a massive bomb plot on home soil?

News of the alleged plot this week has shocked Germans but comes as little surprise to security professionals familiar with a string of cases around the world in which new Muslim converts have embraced the ideology of al Qaeda.

They range from British "shoe bomber" Richard Reid to "American Taliban" John Walker Lindh, jailed in 2002 for fighting alongside the Afghan militia, and Australian David Hicks, the first man convicted of supporting terrorism by a US military court at Guantanamo Bay.

Many analysts believe converts are prone to become extremists as they strain to prove their new-found commitment. Lacking any deep cultural grounding in Islam, they may be more vulnerable to brainwashing with distorted versions of the faith.

"They want to demonstrate in the ranks of militant Islamists that they are professionals dealing with terror and fighting the unbelievers," said Rolf Tophoven, head of Germany's Institute for Terrorism Research and Security Policy.

A Dutch study of 242 Islamist militants arrested in Europe found 14, or 5.8 per cent, were converts. "Converts are new believers, they want to over-compensate their failure to acknowledge God in the past, they are over-eager to become hardcore members," said Edwin Bakker, the report's author.

The two Germans, whose full names have not been made public, were arrested along with a Turkish man on Tuesday in connection with an alleged plot to blow up US-linked targets in Germany.

Authorities said they had stockpiled 730kg of hydrogen peroxide solution – a chemical used in suicide attacks in London in 2005 – and scouted out sites frequented by Americans such as discos, bars and airports.

Interior Minister Wolfgang Schaeuble told Bild newspaper there was "no simple explanation" for the phenomenon of militant converts.

"One would think that someone who grows up here and enjoys the great advantages of our free society is immune. But some are susceptible. These are dangerous, fanatical people with high criminal energy. That worries me a great deal," he said.

Experts say the vast majority of people who convert to Islam do so for legitimate spiritual reasons, drawn by a universal faith that transcends national and racial barriers and provides a new identity, including the choice of a Muslim name.

But case studies show a small minority of converts embrace violent Islamism as an outlet for their alienation and frustration, exploiting the religion as a weapon for attacking society and challenging the existing world order.

M.J. Gohel, security analyst at the Asia-Pacific Foundation in London, said attracting converts was part of a deliberate al Qaeda strategy.

"These individuals have the advantages of being impossible to profile by security services, and with western passports they can travel easily under visa waiver schemes," he said.

"The ideal al Qaeda recruit would be a blue-eyed blonde with an European Union or American passport, who can easily slip through any security radar as he or she would not fit into the current conventional profiling systems."

Al Qaeda also draws propaganda value from converts like California-born Adam Gadahn, known as Azzam the American, who has warned of further attacks on the West in a number of videotaped messages on behalf of Osama bin Laden's network.

At least two converts are believed to have gone on to become senior al Qaeda operatives.

Briton Dhiren Barot, a Hindu by birth, was convicted last year of planning attacks in the United States and Britain using a variety of means including a radiation-laced "dirty bomb".

Christian Ganczarski, a German suspected of involvement in a 2002 bombing in Tunisia, converted at 20 before launching a jihadist career in which investigators believe he became a close associate of bin Laden's.

And not all militant converts are men. Muriel Degauque, 38, a fair-haired, white Catholic-born woman from Belgium, blew herself up in a botched suicide attack in Iraq in November 2005.

- Reuters