WASHINGTON — CIA director Michael Hayden’s explanation today of how videotapes of terror suspect interrogations were made, then destroyed, left many questions unanswered, the chairman of the Intelligence Committee, Jay Rockefeller, said.

The West Virginia Democrat called his panel’s 90-minute closed-door session with Mr. Hayden “a useful and not yet complete hearing” and vowed the committee would get to the bottom of the matter. Among lingering questions: who authorized destruction of the tapes, and why Congress wasn’t told about it.

Mr. Hayden told reporters afterward that he had “a chance to lay out the narrative, the history of why the tapes were destroyed” and the process that led to that decision. But since the taping was done under one of his predecessors, George Tenet, and were destroyed under another, Porter Goss, he wasn’t able to completely answer all questions, he said.

“Other people in the agency know about this far better than I,” Mr. Hayden said, and promised the committee he would make those witnesses available.

The hearing came as a former CIA agent who was part of the interrogation team went public with his account, saying the waterboarding of a top Al Qaeda figure was approved at the top levels of the American government.

According to the former agent, waterboarding of terror suspect Abu Zubaydah got him to talk in less than 35 seconds. The technique, which critics say is torture, probably disrupted “dozens” of planned Al Qaeda attacks, a leader of the team that captured Zubaydah, John Kiriakou, said.

Mr. Kiriakou did not explain how he knew who approved the interrogation technique but said such approval comes from top officials.

“This isn’t something done willy nilly. This isn’t something where an agency officer just wakes up in the morning and decides he’s going to carry out an enhanced technique on a prisoner,” he said today in a round of television news show appearances. “This was a policy made at the White House, with concurrence from the National Security Council and Justice Department.”

At the White House, a spokeswoman, Dana Perino, said the CIA interrogation program approved by the president is safe, tough, effective, and legal. But she said that Mr. Hayden will not “talk about techniques and explain to the enemy what we are doing” during two days of questioning before closed sessions of the Senate and House intelligence panels.

“It’s no secret that the president approved a lawful program in order to interrogate hardened terrorists,” Ms. Perino said. “We do not torture. We also know that this program has saved lives by disrupting terrorist attacks.”

Mr. Kiriakou said that each time CIA agents wished to use waterboarding or any other harsh interrogation technique, they had to present a “well-laid out, well-thought out reason” to top government officials. In Zubaydah’s case, Mr. Kiriakou said the waterboarding had immediate effect.

“The next day, he told his interrogator that Allah had visited him in his cell during the night and told him to cooperate,” Mr. Kiriakou said in an interview first broadcast yesterday evening on ABC News’ World News. “From that day on, he answered every question. The threat information he provided disrupted a number of attacks, maybe dozens of attacks.”

Zubaydah, the first high-value detainee taken by the CIA in 2002, is now being held with other detainees at the American base at Guantanamo Bay, Cuba. He told his interrogators about alleged 9/11 accomplice Ramzi Binalshibh, and the two men’s confessions also led to the capture of Khalid Sheikh Mohammed, whom the American government said was the mastermind behind the September 11 terrorist attacks.

President Bush said he didn’t know about the CIA tapes or their destruction until last week. “My first recollection of whether the tapes existed or whether they were destroyed was when Michael Hayden briefed me,” Mr. Bush said in an interview today with ABC News. “There’s a preliminary inquiry going on and I think you’ll find that a lot more data, facts will be coming out,” the president said. “That’s good. It will be interesting to know what the true facts are.”

Waterboarding is a harsh interrogation technique that involves strapping down a prisoner, covering his mouth with plastic or cloth and pouring water over his face. The prisoner quickly begins to inhale water, causing the sensation of drowning.

The CIA is known to have waterboarded three prisoners — Zubaydah, Khalid Sheik Muhammed, and Abd al-Rahim al-Nashiri, whom the American government says coordinated the 2002 attack on the USS Cole. The CIA has not used the technique since 2003, according to a government official familiar with the program. Mr. Hayden prohibited waterboarding in 2006. The American military outlawed it the same year.

Mr. Hayden told CIA employees last week that the CIA taped the interrogations of two alleged terrorists in 2002. He said the harsh questioning was carried out only after being “reviewed and approved by the Department of Justice and by other elements of the Executive Branch.” Mr. Hayden said Congress was notified in 2003 both of the tapes’ existence and the agency’s intent to destroy them.