Government intelligence officials say terrorists are doing a better job of flying under the radar inside the U.S. Also, a recent spike in the number of plots shows the terrorists have turned to homegrown terrorism to do it.
The threat of another terror attack on U.S. soil has law enforcement officials concerned, but even more disturbing is the growing number of Americans who have been apprehended trying to initiate attacks.
On Wednesday, officials testifying before the Senate Homeland Security and Governmental Affairs Committee said at least 63 U.S. citizens have been charged or convicted of terrorists acts or crimes since 2009.
"That's an astounding high number," said Sen. Joe Lieberman, I-Conn.
"We have seen a dramatic spike," said Sen. Susan Collins, R-Maine. "Do you believe this is an aberration or is this likely to continue?"
"Caution would dictate that we assume this is not an aberration," Homeland Security Secretary Janet Napolitano told the committee.
Terror experts said groups affiliated with al Qaeda are now actively targeting the U.S and they are using Americans who are less likely to be get caught by heightened security measures.
"A new and changing facet of the terrorist threat comes from 'homegrown' terrorists, and by which I mean U.S. persons," Napolitano said.
Since the terrorist attacks on the World Trade Center on Sept. 11, 2001, there have been three serious attempted terror attacks in just seven months.
In September 2009, a failed plot by al Qaeda to set off bombs in New York's subway system was followed by the failed plot by Umar Farouk Abdulmutallab, the "Christmas Day bomber," who was sent to blow up a passenger Northwest airliner on its approach to the Detroit airport, and the Times Square bombing attempt in May by Faisal Shahzad.
Intelligence officials warned the panel that more attacks were likely to come.
"The past year has seen the most significant developments since 9/11," said Michael Leiter, head of the National Counterterrorism Center.
Had these smaller scale attacks been successful, officials said they would have had a severe impact on Americans sense of security
"Launching a larger attack, perhaps more devastating attack, is not worth the additional effort when you can get substantial coverage and impact with smaller attacks," said Robert Mueller, director of the Federal Bureau of Investigation.
Government officials have become increasingly concerned about the role the Internet plays in helping recruit Americans for al Qaeda. Napolitano said U.S.-born, Yemen-based Anwar al-Awlaki is an illustration of an English-speaker spreading propaganda over the Internet, an approach she said could be helping to increase the number of homegrown extremists.