Terrorism experts have puzzled over al-Qaeda's apparent unwillingness after the Sept. 11, 2001, attacks to use car bombs, improvised explosives and small arms to conduct assaults in the United States. The group appeared fixated on orchestrating another dramatic mass-casualty event, such as the simultaneous downing of several commercial airliners.

Indeed, attacks inspired by al-Qaeda in Madrid in 2004 and London in 2005 involved multiple, coordinated bombings targeting mass-transit systems.

But the risk of a single-target bombing or an attack by a lone gunman has increased, officials say, with the rise of al-Qaeda-affiliated groups in the tribal areas of Pakistan, in Yemen and in Somalia, and with the emergence of radicalized Americans inspired by the ideology of violent jihad.

"The impact of the attempted attacks during the past year suggests al-Qaeda, and its affiliates and allies, will attempt to conduct smaller-scale attacks targeting the homeland but with greater frequency," said Michael Leiter, director of the National Counterterrorism Center, pointing to plots against the subway system in New York, the attempt to down a commercial airliner approaching Detroit and the failed car bombing in Times Square.

Leiter said in his testimony that "al-Qaeda in Pakistan is at one of its weakest points organizationally," but he noted that "regional affiliates and allies can compensate for the potentially decreased willingness of al-Qaeda in Pakistan - the deadliest supplier of such training and guidance - to accept and train new recruits."

Officials in the United States and Europe have expressed concern about some of their citizens and residents turning to the Taliban in Pakistan; al-Qaeda in the Arabian Peninsula; and al-Shabab, a militant group in Somalia, for inspiration and training.

"The spike in homegrown violent extremist activity during the past year is indicative of a common cause that rallies independent extremists to want to attack the homeland," said Leiter.

"Key to this trend has been the development of a U.S.-specific narrative that motivates individuals to violence. This narrative - a blend of al-Qaeda inspiration, perceived victimization and glorification of past plotting - has become increasingly accessible through the Internet, and English-language Web sites are tailored to address the unique concerns of U.S.-based extremists."

FBI Director Robert S. Mueller III said it is troubling, and a challenge for investigators, that homegrown extremists have increasingly diverse backgrounds.

"During the past year, the threat from radicalization has evolved," he said. "A number of disruptions occurred involving extremists from a diverse set of backgrounds, geographic locations, life experiences and motivating factors that propelled them along their separate radicalization pathways.

"Beyond the sheer number of disruptions and arrests that have come to light, homegrown extremists are increasingly more savvy, harder to detect and able to connect with other extremists overseas."