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FBI agent Ali Soufan testifies from behind a black curtain and a room divider, right, to protect his identity, on Capitol Hill in Washington on Wednesday. ) ((Manuel Balce Ceneta/Associated Press))

A former FBI agent who interrogated key al-Qaeda suspects shortly after the attacks of Sept. 11, 2001, stopped short of calling waterboarding torture in his testimony before a congressional committee on Capitol Hill on Wednesday.

Because he worked undercover, Ali Soufan had never before spoken publicly about his objections to harsh interrogations techniques such as waterboarding.

Testifying to a Senate panel behind a screen to hide his identity, Soufan said his team's non-threatening interrogation approach elicited crucial information from al-Qaeda operative Abu Zubaydah, including intelligence on "dirty bomb" terrorist Jose Padilla.

"We obtained a treasure trove of highly significant actionable intelligence," he said.

But Soufan said his team had to step aside when CIA contractors took over. They began using harsh methods that caused Zubaydah to "shut down," Soufan said.

"These techniques, from an operational perspective, are slow, ineffective, unreliable, and harmful to our efforts to defeat al-Qaeda."

Soufan said waterboarding and other forms of harsh interrogation are no match for al-Qaeda fighters who are trained to face torture at the hands of the Middle East's most authoritarian regimes.

He said Zubaydah offered no useful information when the interrogation turned harsh. U.S. President Barack Obama has said he wanted to avoid partisan hearings over the interrogations, but the hearing quickly  turned partisan.

In response to Soufan's testimony, Democratic Senator Sheldon Whitehouse read a 2005 statement by then president George Bush that said the opposite — that so-called enhanced interrogation techniques led to important intelligence.

"Does that statement by the president accurately reflect the interrogation of Abu Zubaydah? Based on my recollection he was told probably half truth. And repeated a half truth obviously. Yes sir," replied Soufan.

He stopped short of calling waterboarding and the other controversial U.S. techniques torture.

The justice committee is only part of the Senate investigation into the treatment of detainees as well as the legal reasons behind it.

Senators on both sides say they are just seeking the truth. But if that truth shows laws were broken, it is  unclear whether anyone will be prosecuted.