The Obama administration, grappling with a spate of recent Islamic terrorism cases on U.S. soil, has concluded that the country confronts a rising threat from homegrown extremism.

Anti-terrorism officials and experts see signs of accelerated radicalization among American Muslims, driven by a wave of English-language online propaganda and reflected in aspiring fighters’ trips to hot spots such as Pakistan and Somalia.

Europe had been the front line, the target of successive attacks and major plots, while the U.S. remained relatively calm. But the number, variety and scale of recent U.S. cases suggest 2009 has been the most dangerous year domestically since 2001, anti-terrorism experts said:

* There were major arrests of Americans accused of plotting with Al Qaeda and its allies, including an Afghan American charged in a New York bomb plot described as the most serious threat in this country since the Sept. 11 attacks.

* Authorities tracked other extremism suspects joining foreign networks, including Somali Americans going to the battlegrounds of their ancestral homeland and an Albanian American from Brooklyn who was arrested in Kosovo.

* The FBI rounded up homegrown terrorism suspects in Dallas, Detroit and Raleigh, N.C., saying that it had broken up plots targeting a synagogue, government buildings and military facilities.

Last week, Homeland Security Secretary Janet Napolitano issued her strongest public comments yet on the homegrown threat.

“We’ve seen an increased number of arrests here in the U.S. of individuals suspected of plotting terrorist attacks, or supporting terror groups abroad such as Al Qaeda,” Napolitano said in a speech in New York. “Home-based terrorism is here. And, like violent extremism abroad, it will be part of the threat picture that we must now confront.”

Officials acknowledged that her tone had changed, though they said terrorism has been her focus since becoming Homeland Security chief.

In some of the 2009 cases, extremist leanings are suspected but motives are not known.

Army Maj. Nidal Malik Hasan -- accused of killing 13 people in a Ft. Hood, Texas, shooting rampage last month -- has apparently suffered emotional problems. But in interviews, officials and experts have also raised his Muslim beliefs as an alleged motive.

A previous attack on the U.S. military, a shooting in June by an American convert who killed a soldier and wounded another at an Arkansas recruiting center, was apparently a case of a lone wolf radicalized in Yemen, according to Homeland Security officials.

“You are seeing the full spectrum of the threats you face in terrorism,” former Homeland Security Secretary Michael Chertoff said.

“Radicalization is clearly happening in the U.S.,” said Mitchell Silber, director of analysis for the Intelligence Division of the New York Police Department. “In years past, you couldn’t say that about the U.S. You could say it about Europe.”

Europe has suffered a militant onslaught: transport bombings in Madrid in 2004 and London in 2005, an assassination in the Netherlands in 2004, and close calls such as the fiery failed attack on the Glasgow airport in 2007.

Hard borders have helped the U.S. ward off the threat. But experts also said that Islamic radicalization is more widespread in Europe. Crime, alienation and extremism roil Muslim immigrant communities in places like tiny Denmark and the vast slums of France.

In contrast, American Muslims are wealthier, better educated and better integrated because the United States does a good job of absorbing immigrants and fostering tolerance, experts said. During the last decade, Americans have been a rare presence in the Al Qaeda-connected camps in Pakistan and Afghanistan that have trained hundreds of Westerners and thousands of recruits from Muslim-majority nations.

Nonetheless, recent investigations have run across Americans suspected of being operatives of Al Qaeda and its allies who were trained overseas and, in several cases, allegedly conspired with top terrorism bosses. They include a convert from Long Island, N.Y, who was captured in Pakistan late last year; a Chicago businessman accused of scouting foreign targets for a Pakistani network; and at least 15 Somali American youths from Minneapolis who returned to fight in their ancestral homeland.

“A larger trend has emerged that is not surprising, but is disturbing,” Chertoff said. “You are beginning to see the fruits of the pipeline that Al Qaeda built to train Westerners and send them back to their homelands. . . . This underscores the central significance of disrupting the pipeline at its source.”

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.”

sebastian.rotella@latimes.com