A campaign of U.S. airstrikes launched last year has pounded Al Qaeda hide-outs in Pakistan. But the flow of trainees gathered momentum in 2007 when Pakistani security forces ceded turf to militant groups, officials said. The suspect in the New York plot, Najibullah Zazi, and the Long Island convert, Bryant Neal Vinas, allegedly met in Pakistan in 2008 and discussed attacks on U.S. targets with Al Qaeda chiefs.
Vinas and Zazi are the first Americans to be accused of joining Al Qaeda in several years.
Meanwhile, Silber said in recent congressional testimony: "There have been a half-dozen cases of individuals who, instead of traveling abroad to carry out violence, have elected to attempt to do it here. This is substantially greater than what we have seen in the past, and may reflect an emerging pattern."
Some feel radicalization in the United States has been worse than authorities thought for some time.
"People focused on the idea that we're different, we're better at integrating Muslims than Europe is," said Zeyno Baran, a scholar at the Hudson Institute, a think tank in Washington. "But there's radicalization -- especially among converts [and] newcomers, such as the Somali case shows. I think young U.S. Muslims today are as prone to radicalization as Muslims in Europe."
In proportion to population, extremism still appears less intense in the United States. But the Internet functions as the global engine of extremism. Websites expose Americans to a wave of slick, English-language propaganda from ideologues such as Anwar Awlaki, the Yemeni American described as a spiritual guide for the accused Ft. Hood shooter and other Westerners.
And socioeconomic success will not necessarily prevent Americans' radicalization. Studies suggest that a quest for identity and the bonding process among small groups often drive militants more than personal hardship does.
"The profile in Europe is in general quite different [from U.S. extremists]: more working-class or even underclass," said a European intelligence official who requested anonymity for security reasons. "But it's a bit simplistic to make assumptions. We have seen everything in Europe -- educated people, doctors involved in terrorism. The underclass argument is not enough."
The Obama administration began the year with gestures to the Muslim world. President Obama promised to shut down the prison at Guantanamo Bay, Cuba, and made a historic speech in Cairo.
The Homeland Security Department leads the administration's counter-radicalization effort. The Office of Civil Rights and Civil Liberties, which works with Muslim leaders, held summit meetings with Somali communities this year in Minnesota and Ohio, said David Heyman, assistant Homeland Security secretary for policy.
But that office still lacks a director, critics point out, and the department has yet to fill other key posts as well.
"We don't do enough about fostering a counter-narrative," said Matthew Levitt, a former anti-terrorism official for the Treasury Department now with the Washington Institute for Near East Policy. "Competing for space with the radicalizers and challenging their radical ideologies is the key."
In contrast to the heightened extremist activity in the United States, Europe has remained relatively calm this year. But the West needs to keep up its guard on both sides of the Atlantic, said Farhad Khosrokhavar, an Iranian French scholar who interviewed jailed extremists for his book "Inside Jihadism."
"You can be middle-class and have bright prospects but become a jihadist," he said. "We have to broaden the analysis. This idea of American exceptionalism, the comparison with Europe, should not blind us to the fact that we are going toward a broader participation in jihad."