Battling Terror, Chertoff Felt 'Invisible Burden'

"While we thought the British authorities knew everybody in the conspiracy, we all agreed, both in Britain and here, that there might be some loose ends that we hadn't detected," he said.

The unknown. What's been missed? That is always the worry of U.S. law enforcement and intelligence.

Chertoff said he worries people have become complacent and that the Obama administration should know coming in that al Qaeda remains lethal, and Westerners trained to carry out terror attacks are operating off the radar, still out there.

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Chertoff said there have been successes in "pushing back" against al Qaeda in the tribal regions of Pakistan, where the Westerners are believed to have trained, but that "we still have some unaccounted for, trained terrorists who are somewhere. And we wonder where they're gonna be and what their targets are."

Though, at present, "there is no specific, credible threat of a terrorist attack," Chertoff said a "strategic shift" in focus took place in the department in the summer of 2007 as a result of the activity in Pakistan. The fear is that the new recruits, who could easily blend in with Western society, were taking the first step toward a potential attack on the United States or Europe.

"So, that has remained with us as a concern -- what has happened to those people, what's going on in the pipeline, so to speak, since that summer about 18 months ago," Chertoff said.

Though he would not discuss specific operations, such as the recent uptick in drone attacks in Pakistan, Chertoff said that "if the enemy is worrying about its own safety and security, they're not spending as much time and they're not as effective in plotting attacks against us."

One such strike took out two top al Qaeda figures, U.S. officials confirmed Thursday.

Finding out that information on unaccounted for terror suspects is the crux of a large part of the department's intelligence gathering, Chertoff said, and it's also "why we are working so hard to put additional measures at our border to make it difficult for people to come in."