A nightclub bouncer who reportedly became a terror group leader. A man who tweeted a photo of his young son clutching a severed head. A teenager who is believed to have turned suicide bomber, and others suspected of attempting to travel to Syria to join the Islamic State movement. All of them, Australian. The London-based International Center for the Study of Radicalization and Political Violence reports that between 100 and 250 Australians have joined Sunni militants in Iraq and Syria. Given Australia's vast distance from the region and its population of just 24 million, it is a remarkable number. The center estimates that about 100 fighters came from the United States, which has more than 13 times as many people as Australia. Experts disagree about why Islamic State has been so effective recruiting in Australia, which is widely regarded as a multicultural success story, with an economy in an enviable 24th year of continuous growth. Possible explanations include that some Australian Muslims are poorly integrated with the rest of the country, and that Islamic State recruiters have given Australia particular attention. In addition, the Australian government failed to keep tabs on some citizens who had been radicalized, and moderate Muslims have been put off by some of Prime Minister Tony Abbott's comments about their community. Greg Barton, a global terrorism expert at Monash University in Melbourne, said Australia and some other countries underestimated Islamic State's "pull factor." "We're all coming to terms with the fact that this is a formidable targeter and predatory recruiter that goes after individuals one by one with a very masterful use of technology, and our sense of confidence that because we've got society working well makes us secure misses the point," Barton said. Muslims make up about 2.2 percent of the population in Australia, compared to just 1 percent in the United States. And while many U.S. Muslims are from families who migrated in pursuit of the American economic dream, a larger proportion of Australian Muslims are from families who fled Lebanon's civil war in the 1970s and '80s. Australian Muslims of Lebanese origin are largely based in Sydney, the country's biggest city. They have been less successful in integrating into Australian society than many other groups, and the first Australian-born generation of these migrant families has been overrepresented in terrorism offenses and general street crime. Mohammad Ali Baryalei, an ethnic Lebanese who reportedly became a high-ranking member of Islamic State's operational command, was formerly a Sydney nightclub bouncer and bit-part television actor. Australian security agencies suspect he single-handedly recruited dozens of Australians and helped them enter Syria. Once a Sydney street preacher with the Muslim group Street Dawah, Baryalei was reportedly killed in battle in Syria last fall at age 33. The Australian government has yet to confirm his death. Baryalei is accused in court documents of inciting from afar Islamic State sympathizers in Sydney to brutally slay a randomly selected victim. Security services recorded a telephone conversation between him and Omarjan Azari, who is awaiting trial on charges that include preparing to commit a terrorist act. "What you guys need to do is pick any random unbeliever," Baryalei allegedly told Azari, according to court testimony. "Backpacker, tourist, American, French or British, even better."