A new study on homegrown terror found that most American Muslims who planned violent attacks in since 2001 were young, male U.S. citizens who became radical as part of a group.

Still, researchers seeking lessons on preventing extremism found no definitive pattern of how the suspects turned to violence and no geographic center of radicalization in the U.S.

Experts from Duke University and the University of North Carolina at Chapel Hill tallied homegrown terror cases since the Sept. 11 attacks and found 139 American Muslims had been publicly accused of planning or carrying out violence motivated by extremism.

All but one of the suspects were male and most were under age 30. Most were U.S.-born, naturalized citizens or legal residents of the country. Although Arabs formed the largest group of suspects, the accused were almost evenly divided in terms of ethnicity, including African-Americans, South Asians, Somalis and whites. About a third were converts to Islam.

Funded by the National Institute of Justice, a division of the U.S. Justice Department, researchers sought to learn why American Muslims seem less prone to extremism than Muslims in Western Europe, where radicals preach openly and children and grandchildren of Muslim immigrants often feel as alienated from broader society as their parents did.

The report’s authors analyzed public records of terror cases, reviewed efforts by American Muslim leaders to fight extremism, and interviewed more than 120 Muslims in Houston; Seattle; Buffalo, N.Y.; and around Raleigh and Durham in North Carolina. Each of the four areas had some cases of alleged radicalization. U.S. Muslims accused of sending money to overseas terrorist groups were not part of the study.

Researchers found the largest number of homegrown terror cases occurred last year, with a total of 41 suspects, although the report’s authors say it’s too early to know if that is an aberration or a trend. The 2009 increase is partly due to the cases of young Somali-Americans in Minneapolis believed to have joined Somalia’s al-Shabab jihadist, or holy war, movement.

The cases also include Army Maj. Nidal Hasan, charged with the Fort Hood mass shooting last November, and the five young men from Virginia who were recently arrested in Pakistan, allegedly on their way to get terrorist training and join the Taliban in Afghanistan.

The study found that the planned targets of most violent plots were overseas. Seventy percent of the conspiracies were pre-empted by law enforcement well before anyone was hurt.

Maria Haberfeld, a professor at the John Jay College of Criminal Justice who studies policing and counterterrorism, called terrorism “a fluid and evolving phenomenon” and said she wasn’t surprised that no clear pattern of radicalization emerged, especially from the report’s small sample.

“When you look at various individuals who were involved more recently in terrorist attacks, everybody came from a different background, whether economically or educationally. Everybody has some issue, but we’re not sure what that issue was. Mental illness? Ideology? Some temporary frame of mind?” she said. “It’s very situational. Something triggers certain sentiments and reactions that there is no way we could have predicted.”

Muqtedar Khan, a University of Delaware political scientist, said “a small minority of radicalization” has occurred within the American Muslim community, but said it’s not clear why.

Anger over U.S. foreign policy is generally considered a factor, as is a feeling of alienation due to intense suspicion of Muslims in the aftermath of the Sept. 11 strikes.

The study was released amid the uproar over national security following the attempt by a Nigerian to allegedly destroy a Detroit-bound U.S. airliner on Christmas Day.

The report, called “Anti-Terror Lessons of Muslim-Americans,” credited U.S. Muslim leaders with vigorously monitoring their communities for potential threats. The study’s authors urged civil authorities to offer more support for projects, such as Muslim youth groups, that reinforce the message that extremism is contrary to Islam.

Khan said the American Muslim community has developed good relationships with law enforcement, even with increased tensions in the last year over the FBI’s use of confidential informants in mosques. The community has driven radical speakers from mosques and national organizations, Khan said. However, he said, “people don’t have to go to mosques to get radicalized” and the American Muslim community isn’t equipped right now to counter extreme messages over the Internet and through the media.