Mohammad Hassan Khalid, a Pakistani teen who grew up in Ellicott City, was indicted last year on charges of providing material support to terrorists.

Khalid, 18, is accused of using the Internet to help create a "a violent jihad organization." Authorities say he worked with the suburban Philadelphia woman known as "Jihad Jane" to recruit Americans and Europeans who would be divided into teams and assigned tasks that included planning, research, finance and action.

Khalid, who graduated from Mount Hebron High School last year with plans to enroll at the Johns Hopkins University, has pleaded not guilty.

The number of Americans who have been radicalized since Sept. 11, 2001, remains small. Brian Michael Jenkins, a senior scholar at the Rand Corp., counts fewer than 200 cases among a Muslim community estimated at 3 million in the United States.

"The jihadist ideology is simply not gaining any foothold among American Muslims," Jenkins said. "It's rejected."

Still, homegrown terrorists, because they know the country and its vulnerabilities, and move freely and undetected through society, pose a distinct threat.

Most notoriously, Army Maj. Nidal Hassan is accused of killing 13 soldiers and civilians at Fort Hood, Texas, and wounding 32 more when he opened fire with a semiautomatic pistol in November 2009.

The potentially more lethal plot of Times Square bomber Faizal Shahzad, a Pakistani-American, fizzled not because it was thwarted by authorities, but because his car bomb failed to detonate.

Investigators and analysts say the radicalized fit no single profile. They have included young and old, men and women, natives and immigrants, rich and poor, college graduates and high school dropouts, lifelong Muslims and recent converts.

Compounding the challenge for authorities, they are often isolated from their communities and work and plot alone.

"Most of the cases, more than two-thirds, involve a single individual," Jenkins said. "Their actions are invariably a matter of self-initiative. … And it's very, very difficult to pick them up because they don't necessarily hit any of the tripwires that might bring them to the attention of authorities."

One common theme does emerge. While many are isolated within their communities, most are well-connected online, where al-Qaida and its affiliates are expanding their activities.

"Whereas the Internet was previously used to spread propaganda, it is now used in recruiting, radicalizing, training and inciting terrorism," Mark F. Giuliano, assistant director of the FBI Counterterrorism Division, said last year. "Thousands of extremist websites promote violence to a worldwide audience predisposed to the extremist message."

Jenkins says the shift to the virtual battlefield is partly a result of U.S. success on real ones.

"These groups no longer have the capacity that al-Qaida had in 2001 for planning and preparing and executing these ambitious, centrally directed attacks," he said. "As we have degraded those operational capacities, they have put greater emphasis on a do-it-yourself strategy — that is, greater efforts on communications, on reaching people through the Internet and persuading them to do whatever they can wherever they are."

The United States is working to show up in the same places to rebut terrorists' claims. The State Department established the Center for Strategic Counterterrorism Communications in 2010 to develop, in the words of Ambassador Richard LeBaron, "narratives and public communications strategies to confront and discredit the extremist messages."

Speaking recently at the Washington Institute for Near East Policy, LeBaron said the center is addressing a "specific, narrowly defined" audience: "People who are sympathetic to the views of al-Qaida and could be vulnerable to its propaganda; people who could be persuaded or enticed into crossing the boundary between sympathy and action, until they pick up a gun or strap on a bomb or directly facilitate an attack.

"Our job is to nudge people into a different path; help them question some of their assumptions; and contribute to an environment in which terrorist violence is not considered a viable, acceptable or effective option."

That work includes "digital outreach" to challenge "extremist messages online in Arabic, Urdu and Somali through participation on forums, blogs, media and social-networking sites," LeBaron said.

The U.S. government hasn't always been comfortable with such outreach, says Matthew Levitt, director of the Stein Program on Counterterrorism and Intelligence at the Washington Institute for Near East Policy.