Please read Steven Emerson’s September 20, 2006 Congressional testimony, "The Homeland Security Implications of Radicalization." It is a concise summary of the process and fruits of radicalization here in the US. It includes excellent open source summaries of US-based cases, including the Lackawanna Six, the Virginia Jihad Network, Abu Ali, and of course, Adam Gadahn:
Adam Gadahn, a convert to Islam, grew up on a farm in California. He was born Adam Pearlman to a Catholic mother and a Jewish father who later converted to Christianity, taking the name Gadahn. As a young man, he was interested in death-metal music and hosted a show on the environment on a student television station. In 1997, at the age of 17, he converted to Islam under the tutelage of a purportedly moderate religious leader named Haitham "Danny" Bundakji and was hired as a security guard at the Islamic Society of Orange County. Bundakji claimed that Gadahn was then befriended by a group of Pakistani nationals he described as “fundamentalist” who were outspoken in their criticism of moderation and Bundakji’s interfaith activities, calling him “Danny the Jew.” One of the group was Hisham Diab, a well-connected al Qaeda operative who once hosted the blind sheik Omar Abdel Rahman at his home. After Bundakji banned these men from the mosque, Gadahn stormed angrily into Bundakji’s office, slapped him in the face, and accused him of not being a true Muslim. Shortly after this incident, Gadahn left for Pakistan and kept in touch with his family only occasionally.
And there’s this excellent synopsis of the Internet’s role in the global jihad:
The U.S. led invasion of Afghanistan in response to the attacks of September 11, 2001 forced an historically and strategically significant shift on the part of al Qaeda that reverberated throughout the larger jihadi movement. The successful invasion decimated the hierarchy and configuration of al Qaeda, which was centralized in Afghanistan. Al Qaeda was forced to devolve to an ideological presence and surrender the greater portion of operational control outward to various affiliate groups. While these affiliate groups continued to direct jihad around the world, the ideology of al Qaeda continued to spread and led to the formation of various provisional cells, several of which have been homegrown. Instead of a centralized organization, al Qaeda has become a franchised idea. While many prominent jihadist thinkers agitated over the circumstances that forced this strategic shift, some – such as Mustafa Setmarian Nasar, popularly known as Abu Musab al-Suri – had promoted the strategic necessity of this change for the wider Salafi jihadist movement for some time.
Emerson writes with the confidence and precision that comes with knowing the subject better than practically anyone else inside or outside the intelligence community. In a few pages he crystallizes the nature and scope of the threat.