The country is also home to radical Islamic cleric Anwar al Awlaki, a Yemeni-American who has used the Internet to encourage attacks from abroad.
Asked by chairman King to compare Osama bin Laden and Awlaki, Leiter said he considers Awlaki and the group al Qaeda in the Arabian Peninsula, which he leads, "probably the most significant risk to the U.S. homeland."
Awlaki had ties to the attempted 2009 Christmas day bombing of Northwest flight 253 and last fall's cargo bomb plot, Adam Gadahn, a top propaganda master for al Qaeda, and Omar Hammami, a U.S. citizen from Alabama turned top recruiter and lieutenant in the al Qaeda-linked Al Shabaab terrorist network in Somalia.
"He's an extremely dangerous man. He has shown a desire to harm the United States, a desire to strike the homeland of the United States," U.S. Attorney General Eric Holder said in December. "He is a person who -- as an American citizen -- is familiar with this country and he brings a dimension, because of that American familiarity, that others do not."
Holder said that as a threat to the United States, Awlaki ranks alongside bin Laden.
"He would be on the same list with bin Laden," the attorney general said. "He's up there. I don't know whether he's one, two, three, four -- I don't know. But he's certainly on the list of the people who worry me the most."
Napolitano and Leiter sought to reassure members of Congress and the public today that the agencies were actively working to counter the evolving dangers, including Awlaki.
Leiter said the agency's new "pursuit groups," created after intelligence officials failed to connect the dots and detect the 2009 Christmas Day bomb plot, have "repeatedly identified key leads that would have otherwise been missed amidst a sea of uncorrelated data."
He also said the government has bolstered its terrorist watch lists and enhanced its search capability with a "Google-like" function. Previously, the databases could not be quickly searched or cross-referenced.
Authorities said that while the threat of homegrown terrorism remains acute, the capabilities of al Qaeda have been successfully degraded to "one of its weakest points in the past decade."
"During the past two years, al Qaeda's base of operations in the Federally Administered Tribal Areas [of Pakistan] has been restricted considerably, limiting its freedom of movement and ability to operate," Leiter said. "The group has been forced to react continuously to personnel losses that are affecting the group's morale, command and control, and continuity of operations."