'Plot Would Have Killed Thousands'

That progress was aided after the arrests last year that provided Homeland Security with information about terrorist capabilities.

"Clearly, the effort to put explosives in sports bottles was a reaction to what we had done with respect to other kinds of explosives, and … we're going to be back and forth with terrorists on this kind of cat-and-mouse process for years to come," Chertoff said.

And while he is confronted by pieces of data daily as Homeland Security tries to assess credible threats and piece together information, Chertoff said he remains continually struck by the nature of the enemy.

"You know, we go about our business during the summer, other times of the year. People are going to ballgames or watching their children graduate from high school," he said, "and it chills me sometimes to think there are people a half a world away who are spending the same period of time in a cave, trying to figure out how to kill us."