"Any American who joins al Qaeda will know full well that they have joined an organization that is at war with the United States," Brennan said. "Any American who did that should know well that they in fact are part of an enemy ... and that the U.S. will do anything that is possible to destroy that enemy to save American lives."
Senators also asked Brennan whether the CIA's enhanced interrogation program, to which Brennan has been linked, yielded intelligence that led to the killing of Osama bin Laden. Committee Democrats, who released a 6,000 page report on the matter, have protested that enhanced interrogations did not yield meaningful intelligence.
After reading the 300-page summary of that report, Brennan said he's not sure.
"There clearly were a number of things, many things that I read in that report that are very concerning and disturbing to me, and ones that I would want to look into immediately," Brennan told Feinstein.
Brennan said he had previously thought enhanced interrogations helped in the bin Laden search, based on information he was given at the time. Brennan told one senator that the report has cast doubt on what he was told.
"At this point, senator," Brennan said, "I do not know what the truth is."
Pressed by Sen. Carl Levin on the practice of waterboarding, Brennan would not say whether or not he believes it qualifies as "torture," but Brennan did express disgust with the practice and said it would not occur under his watch at the CIA.
"It is reprehensible and something that should not be done," Brennan said. "Waterboarding is something that should have never been employed, and as far as I'm concerned, something that never would be if I were confirmed."
Brennan also warned of catastrophic consequences of the looming budget sequestration, which will happen on March 1 unless Congress agrees to deficit-reduction measures or passes a temporary fix.
"It's going to have a devastating impact on the national security of this country," Brennan said.