Senate Democrats demanded a criminal investigation into waterboarding by government interrogators after the Bush administration admitted for the first time that the tactic was used on three terror suspects.
In congressional testimony, CIA director Michael Hayden became the first administration official to publicly acknowledge the agency used waterboarding on detainees following the September 11 2001, terrorist attacks.
Waterboarding involves strapping a suspect down and pouring water over his cloth-covered face to create the sensation of drowning. It has been traced back hundreds of years, to the Spanish Inquisition, and is condemned by nations around the world.
"We used it against these three detainees because of the circumstances at the time," Mr Hayden told the Senate Intelligence Committee.
"There was the belief that additional catastrophic attacks against the homeland were inevitable. And we had limited knowledge about al Qaida and its workings. Those two realities have changed."
Hayden said Khalid Sheikh Mohammed, Abu Zubayda and Abd al-Rahim al-Nashiri were waterboarded in 2002 and 2003. Mr Hayden banned the technique in 2006, but National Intelligence Director Mike McConnell told senators during the same hearing yesterday that waterboarding remained in the CIA arsenal - so long as it had the specific consent of the president and legal approval of the attorney general.
That prompted Senator Dick Durbin, the Senate's number two Democrat and a member of the Judiciary Committee, to call on the Justice Department to open a criminal inquiry into whether past use of waterboarding broke any law.
The Pentagon has banned its employees from using waterboarding to extract information from detainees and FBI Director Robert Mueller said his investigators did not use coercive tactics in interviewing terror suspects.
Senator Durbin, already frustrated with attorney general Michael Mukasey's refusal last week to define waterboarding as a form of torture - as critics have - said he would block the nomination of the Justice Department's second in command if the criminal inquiry was not opened.
It was a particularly sharp threat by Senator Durbin, who represents Illinois - the same state that US District Judge Mark Filip of Chicago, the deputy attorney general nominee, calls home.
"In light of the Justice Department's continued non-responsiveness to Congress on the issue of torture, including your disappointing testimony on waterboarding last week, I have reluctantly concluded that placing a hold on Judge Filip's nomination is my only recourse for eliciting timely and complete responses to important questions on torture," Senator Durbin wrote in a letter to Mr Mukasey yesterday.
He added: "A Justice Department investigation should explore whether waterboarding was authorised and whether those who authorised it violated the law."
Justice Department spokesman Brian Roehrkasse declined to comment except to say that the department "is reviewing the letter carefully".
Human Rights Watch, which has been calling on the US government to outlaw waterboarding as a form of illegal torture, called Mr Hayden's testimony "an explicit admission of criminal activity".
Joanne Mariner, the group's counterterrorism director, said it "gives the lie" to the administration's claims that the CIA had not used torture. "Waterboarding is torture, and torture is a crime," she said.
Critics say waterboarding has been outlawed under the United Nations' Convention Against Torture, which prohibits treatment resulting in long-term physical or mental damage.
They also say it should be recognised as banned under the US 2006 Military Commissions Act, which prohibits treatment of terror suspects that is described as "cruel, inhuman and degrading". The act, however, does not explicitly ban waterboarding by name.
During his own Senate appearance last week, Mr Mukasey refused to declare waterboarding illegal, prompting Democrats to accuse him of potentially allowing the harsh interrogation tactic to be used in the future.