The threat posed by homegrown extremists shows that the battle against terrorism has become more complex in the past year, underscoring the challenges of pinpointing and blocking terrorist plots, said Michael Leiter, director of the National Counterterrorism Center.
"Groups affiliated with al Qaeda are now actively targeting the United States and looking to use Americans or Westerners who are able to remain undetected by heightened security measures," FBI Director Robert Mueller told the Senate Homeland Security and Governmental Affairs Committee.
"It appears domestic radicalization and homegrown extremism is becoming more pronounced," Mueller said.
Homeland Security Secretary Janet Napolitano said al Qaeda has inspired an array of terrorist groups.
"We are all seeing more diverse activity" by a more diverse collection of groups, Napolitano said.
Leiter said al Qaeda in Pakistan is at one of its weakest points organizationally. Nonetheless, he said, the terrorist group remains a capable and determined enemy that has proven its resilience over time.
A year ago, the FBI arrested Michael C. Finton in Illinois and Hosam Smadi in Texas in connection with unrelated bomb attempts. The bureau used online undercover agents and confidential human sources who monitored Finto and Smadi until their arrests.
"The threat is still all too real," said Sen. Joe Lieberman, I-Conn., the chairman of the committee.
The panel's ranking Republican, Sen. Susan Collins of Maine, said the shootings at Fort Hood, Texas, and the attempted Christmas Day attack on a Detroit-bound airliner show that the terrorist threat "is evolving and ever-changing" and "a chameleon by design."
Terrorists "are working increasingly to build alliances or essentially recruit soldiers for their army from within the United States," said Lieberman.