His ABC interview came at an especially delicate juncture in the debate over the use of torture. Weeks earlier, the nomination of Michael Mukasey as attorney general was nearly derailed by his refusal to comment on the legality of waterboarding, and one day later, the C.I.A. director testified about the destruction of interrogation videotapes. Mr. Kiriakou told MSNBC that he was willing to talk in part because he thought the C.I.A. had “gotten a bum rap on waterboarding.”

At the time, Mr. Kiriakou appeared to lend credibility to the prior press reports that quoted anonymous former government employees who had implied that waterboarding was used sparingly. In late 2007, Mr. Ross began pursuing Mr. Kiriakou for an interview, “leaning on him pretty hard,” he recounted.

On Dec. 10, in the subsequent interview, Mr. Kiriakou told Mr. Ross that he believed the waterboarding was necessary in the months after the 9/11 attacks. “At the time I was so angry,” he told Mr. Ross. “I wanted so much to help disrupt future attacks on the United States that I felt it was the only thing we could do.”

Mr. Kiriakou was the only on-the-record source cited by ABC. In the televised portion of the interview, Mr. Ross did not ask Mr. Kiriakou specifically about what kind of reports he was privy to or how long he had access to the information. “It didn’t even occur to me that they’d keep doing” the waterboarding, Mr. Ross said last week. “It doesn’t make any sense to me.”

He added, “I didn’t give enough credit to the fiendishness of the C.I.A.”

Mr. Kiriakou refused an interview request last week. In a statement to ABC, he said he was aware only of Mr. Zubaydah’s being waterboarded “on one occasion.”

The C.I.A., which considered legal action against Mr. Kiriakou for divulging classified information, said last week that he “was not — and is not — authorized to speak on behalf of the CIA.”

Paul Gimigliano, a C.I.A. spokesman, said: “This agency did not publicly disclose the frequency with which the waterboard was used, noting only that it was employed with three detainees. If reporters got that wrong, they weren’t misled from here.”