WASHINGTON (CNN)  -- The contentious debate over so-called enhanced interrogation techniques took center stage Wednesday on Capitol Hill as a former FBI agent involved in the questioning of terror suspects testified that such tactics -- including waterboarding -- are ineffective.



From left: Sens. Lindsey Graham, Sheldon Whitehouse,

Patrick Leahy and Dianne Feinstein listen Wednesday.



Ali Soufan, an FBI special agent from 1997 to 2005, told members of a key Senate Judiciary subcommittee that such "techniques, from an operational perspective, are ineffective, slow and unreliable and harmful to our efforts to defeat al Qaeda."



Soufan's remarks followed heated exchanges between committee members with sharply differing views on both the value of the techniques and the purpose of the hearing.



Soufan, who was involved in the interrogation of CIA detainee Abu Zubaydah, took issue with former Vice President Dick Cheney, who has said that enhanced interrogation techniques helped the government acquire intelligence necessary to prevent further attacks after September 11, 2001.



The techniques, which the Bush administration approved, are considered torture by many critics.  Watch analysts discuss harsh interrogations and torture »



"From my experience -- and I speak as someone who has personally interrogated many terrorists and elicited important actionable intelligence -- I strongly believe that it is a mistake to use what has become known as the 'enhanced interrogation techniques,' " Soufan noted in his written statement.



Such a position is "shared by many professional operatives, including the CIA officers who were present at the initial phases of the Abu Zubaydah interrogation."



Soufan told the committee that within the first hour of his interrogation of Zubaydah, the suspected terrorist provided actionable intelligence.



But once the CIA contractors took over and used harsh methods, Zubaydah stopped talking, Soufan said. When Soufan was asked to resume questioning, Zubaydah cooperated. After another round of more coercive techniques used by the contractors, however, Soufan said it was difficult for him to re-engage Zubaydah.



One of four recently released Bush administration memos showed that CIA interrogators used waterboarding at least 266 times on Zubaydah and Khalid Shaikh Mohammed, the suspected planner of the September 11 attacks.



"People were given misinformation, half-truths and false claims of successes; and reluctant intelligence officers were given instructions and assurances from higher authorities," Soufan testified.



"I wish to do my part to ensure that we never again use these ... techniques instead of the tried, tested and successful ones -- the ones that are also in sync with our values and moral character. Only by doing this will we defeat the terrorists as effectively and quickly as possible."  Watch as Soufan makes his case before the panel »



Soufan was hidden behind a protective screen during his testimony before the Judiciary Committee's Subcommittee on Administrative Oversight and the Courts. Staffers for the committee cited "documented threats" against him, noting his previous interaction with al Qaeda terrorists as well as his undercover work against Islamic extremists.



Philip Zelikow, who was a top aide to former Secretary of State Condoleezza Rice, repeated an accusation during the hearing that Bush officials ordered his memo arguing against waterboarding to be destroyed.



The order, "passed along informally, did not seem proper, and I ignored it," Zelikow said.



He said that his memo has been in State Department files and is being reviewed for possible declassification.



Zelikow slammed the "collective failure" behind the government's adoption of "an unprecedented program of coolly calculated dehumanizing abuse and physical torment to extract information. This was a mistake, perhaps a disastrous one."



He added that some "may believe that recent history, even since 2005, shows that America needs an elaborate program of indefinite secret detention and physical coercion in order to protect the nation. ... If they are right, our laws must change and our country must change. I think they are wrong."  Watch Zelikow tell CNN's Kiran Chetry how U.S. leaders came to the wrong conclusions on interrogation policy »



Committee Republicans warned that the hearing ultimately could contribute to diminished national security.



"As we harshly judge those who had to make decisions we
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question: what did the republican committee say?, answer: the hearing ultimately could contribute to diminished national security. | question: What did Committee Republians say?, answer: Republicans warned that the hearing ultimately could contribute to diminished national security. | question: Who did the ex-FBI agent take issue with?, answer: former Vice President Dick Cheney, | question: Who said "such tactics helped U.S."?, answer: Vice President Dick Cheney, | question: What were people given?, answer: misinformation, half-truths and false claims of successes; | question: Who OK'd the techniques used?, answer: the Bush administration | question: What could help diminish national security?, answer: the hearing