The concern comes at a time when anti-terrorism officials and experts have pointed to signs of accelerated radicalization among American Muslims. The trend is driven by a wave of English-language online propaganda that appeals to U.S. extremists and is reflected in increased travel by aspiring fighters to training and combat hot spots such as Pakistan and Somalia. Until recently, Europe has been the front line for Islamic terrorist activity, suffering successive attacks and major plots, while the U.S. remained relatively calm. But the number, variety and seriousness of recent U.S. cases make 2009 seem the most dangerous year in this country since 2001, anti-terrorism officials and experts say. There were major arrests of Americans accused of plotting with al-Qaida and its allies, including an Afghan American charged in a New York bomb plot described as the most serious threat in the United States since the Sept. 11 attacks. Authorities tracked other accused militants joining foreign networks, from Somali Americans returning to the battlegrounds of their ancestral homeland to an Albanian American from New York who was arrested in Kosovo. The FBI rounded up suspected home-grown extremists in cities including Dallas, Detroit and Raleigh, N.C., breaking up alleged plots targeting a synagogue, government buildings and military facilities. Homeland Security Secretary Janet Napolitano warned of the home-grown threat last week in her strongest public comments yet on the issue. '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-Qaida,' 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.' Although officials said Napolitano has focused on terrorism since becoming homeland security chief, they acknowledged a marked change in tone. Extremist leanings are suspected in the prosecutions brought in 2009, but not all the alleged motives are known. In the most dramatic case, Maj. Nidal Malik Hasan -- accused of killing 13 people in a Fort Hood, Texas, shooting rampage last month -- apparently suffered from emotional problems. But officials and experts said in interviews that they believed he also was driven by religious belief. 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 extremist radicalized in Yemen, according to officials. 'You are seeing the full spectrum of the threats you face in terrorism,' former Homeland Security Secretary Michael Chertoff said. Searching for causes of the apparent upsurge in terrorist activity in the United States, officials said intensified radicalization among American Muslims was a key factor. '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.' Although al-Qaida has failed to strike America since 2001, 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, Scotland, airport in 2007.Hard borders have helped the United States 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 such as 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 past decade, Americans have been a rare presence in the al-Qaida-connected camps in Pakistan and Afghanistan that have trained hundreds of Westerners and thousands of recruits from Muslim nations. Nonetheless, recent investigations have run across suspected American operatives of al-Qaida and its allies who were trained overseas and, in several cases, allegedly conspired with top terrorist bosses. They include a Latino 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-Qaida 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-Qaida hide-outs in Pakistan. But the flow of trainees gathered momentum in 2007, when Pakistani security forces had ceded turf to militant groups, officials said. Despite the obstacles, the suspect in the New York plot, Najibullah Zazi, and the Latino convert, Bryant Neal Vinas, allegedly converged in Pakistan in 2008 and discussed attacks on U.S. targets with al-Qaida chiefs. Vinas and Zazi are the first Americans to be accused of joining al-Qaida in several years. 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 Turkish American scholar at the Hudson Institute. '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.' The Obama administration began the year with gestures to the Muslim world. President Barack 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 critics point out that the department has yet to fill key posts, including the director of the Civil Rights and Civil Liberties office. 'We don't do enough about fostering a counternarrative,' said Matthew Levitt, a former anti-terrorism official for the Treasury Department who now is with the Washington Institute for Near East Policy. 'Competing for space with the radicalizers and challenging their radical ideologies is the key.' 