North Carolina ? 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.